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x_transformer_1_23_2.py

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+#===================================================================================================================
+#
+# X Trasformer Module
+#
+# Partial x-transformers code With useful modifications
+#
+# Version 1.0
+#
+# Original source code courtesy of lucidrains
+# https://github.com/lucidrains/x-transformers
+#
+# Original source code retrieved on 10/10/2023
+#
+# Project Los Angeles
+# Tegridy Code 2023
+
+#===================================================================================================================
+
+# Critical dependencies
+#
+# !pip install torch
+# !pip install einops
+
+#===================================================================================================================
+
+from functools import partial
+from typing import Optional, Tuple
+
+import os
+os.environ['USE_FLASH_ATTENTION'] = '1'
+
+import torch
+from torch import nn, einsum, Tensor
+import torch.nn.functional as F
+
+# Flash attention
+from torch.nn.attention import SDPBackend, sdpa_kernel
+torch.backends.cuda.enable_flash_sdp(True)
+
+from collections import namedtuple
+from functools import wraps
+from packaging import version
+from dataclasses import dataclass
+
+from einops import rearrange, repeat
+
+# constants
+
+EfficientAttentionConfig = namedtuple('EfficientAttentionConfig', ['enable_flash', 'enable_math', 'enable_mem_efficient'])
+
+@dataclass
+class Intermediates:
+    qk_similarities: Optional[Tensor] = None
+    pre_softmax_attn: Optional[Tensor] = None
+    post_softmax_attn: Optional[Tensor] = None
+    cached_kv: Optional[Tuple[Tensor, Tensor]] = None
+
+    def to_tuple(self):
+        return (self.qk_similarities, self.pre_softmax_attn, self.post_softmax_attn)
+
+# helpers
+
+def exists(val):
+    return val is not None
+
+def default(val, d):
+    return val if exists(val) else d
+
+def compact(arr):
+    return [*filter(exists, arr)]
+
+def once(fn):
+    called = False
+    @wraps(fn)
+    def inner(x):
+        nonlocal called
+        if called:
+            return
+        called = True
+        return fn(x)
+    return inner
+
+print_once = once(print)
+
+# functions for creating causal mask
+# need a special one for onnx cpu (no support for .triu)
+
+def create_causal_mask(i, j, device):
+    return torch.ones((i, j), device = device, dtype = torch.bool).triu(j - i + 1)
+
+def onnx_create_causal_mask(i, j, device):
+    r = torch.arange(i, device = device)
+    causal_mask = rearrange(r, 'i -> i 1') < rearrange(r, 'j -> 1 j')
+    causal_mask = F.pad(causal_mask, (j - i, 0), value = False)
+    return causal_mask
+
+# main class
+
+class Attend(nn.Module):
+    def __init__(
+        self,
+        *,
+        dropout = 0.,
+        causal = False,
+        heads = None,
+        talking_heads = False,
+        sparse_topk = None,
+        scale = None,
+        qk_norm = False,
+        flash = False,
+        add_zero_kv = False,
+        onnxable = False
+    ):
+        super().__init__()
+        self.scale = scale
+        self.qk_norm = qk_norm
+
+        self.causal = causal
+        self.create_causal_mask = onnx_create_causal_mask if onnxable else create_causal_mask
+
+        self.attn_fn = partial(F.softmax, dtype = torch.float32) if not qk_norm else F.softmax
+
+        self.dropout = dropout
+        self.attn_dropout = nn.Dropout(dropout)
+
+        # talking heads
+
+        assert not (flash and talking_heads), 'talking heads not compatible with flash attention'
+
+        self.talking_heads = talking_heads
+        if talking_heads:
+            self.pre_softmax_talking_heads = nn.Conv2d(heads, heads, 1, bias = False)
+            self.post_softmax_talking_heads = nn.Conv2d(heads, heads, 1, bias = False)
+
+        # sparse topk
+
+        assert not (flash and sparse_topk), 'sparse topk not compatible with flash attention'
+        self.sparse_topk = sparse_topk
+
+        # add a key / value token composed of zeros
+        # in case this helps controlling outliers, proposed by https://www.evanmiller.org/attention-is-off-by-one.html
+
+        self.add_zero_kv = add_zero_kv
+
+        # flash attention
+
+        self.flash = flash
+        assert not (flash and version.parse(torch.__version__) < version.parse('2.0.0')), 'in order to use flash attention, you must be using pytorch 2.0 or above'
+
+        # determine efficient attention configs for cuda and cpu
+
+        self.cpu_config = EfficientAttentionConfig(True, True, True)
+        self.cuda_config = None
+
+        if not torch.cuda.is_available() or not flash:
+            return
+
+        device_properties = torch.cuda.get_device_properties(torch.device('cuda'))
+
+        major, minor = device_properties.major, device_properties.minor
+
+        if (major, minor) == (8, 0):
+            print_once('A100 GPU detected, using flash attention if input tensor is on cuda')
+            self.cuda_config = EfficientAttentionConfig(True, False, False)
+        elif (major, minor) == (9, 0):
+            print_once('H100 GPU detected, using flash attention')
+            self.cuda_config = EfficientAttentionConfig(True, False, False)
+        else:
+            print_once('Non-A100 GPU detected, using math or mem efficient attention if input tensor is on cuda')
+            self.cuda_config = EfficientAttentionConfig(False, True, True)
+
+    def flash_attn(
+        self,
+        q, k, v,
+        mask = None,
+        attn_bias = None
+    ):
+        batch, heads, q_len, _, k_len, is_cuda, device = *q.shape, k.shape[-2], q.is_cuda, q.device
+
+        # Recommended for multi-query single-key-value attention by Tri Dao
+        # kv shape torch.Size([1, 512, 64]) -> torch.Size([1, 8, 512, 64])
+
+        if k.ndim == 3:
+            k = rearrange(k, 'b ... -> b 1 ...').expand_as(q)
+
+        if v.ndim == 3:
+            v = rearrange(v, 'b ... -> b 1 ...').expand_as(q)
+
+        # handle scale - by default they scale by dim_head ** -0.5, but need to take care if using cosine sim attention
+
+        if self.qk_norm:
+            default_scale = q.shape[-1] ** -0.5
+            q = q * (self.scale / default_scale)
+
+        # Check if mask exists and expand to compatible shape
+        # The mask is B L, so it would have to be expanded to B H N L
+
+        causal = self.causal
+
+        # in the case of kv caching with one token (q_len == 1), just turn off causal masking
+        # in speculative decoding, this may go up to 5-6, so right aligned causal mask will be needed there
+
+        if q_len == 1 and causal:
+            causal = False
+
+        # expand key padding mask
+
+        if exists(mask):
+            assert mask.ndim == 4
+            mask = mask.expand(batch, heads, q_len, k_len)
+
+        # handle kv cache - this should be bypassable in updated flash attention 2
+
+        if k_len > q_len and causal:
+            causal_mask = self.create_causal_mask(q_len, k_len, device = device)
+            if not exists(mask):
+                mask = ~causal_mask
+            else:
+                mask = mask & ~causal_mask
+            causal = False
+
+        # manually handle causal mask, if another mask was given
+
+        row_is_entirely_masked = None
+
+        if exists(mask) and causal:
+            causal_mask = self.create_causal_mask(q_len, k_len, device = device)
+            mask = mask & ~causal_mask
+
+            # protect against an entire row being masked out
+
+            row_is_entirely_masked = ~mask.any(dim = -1)
+            mask[..., 0] = mask[..., 0] | row_is_entirely_masked
+
+            causal = False
+
+        # handle alibi positional bias
+        # convert from bool to float
+
+        if exists(attn_bias):
+            attn_bias = rearrange(attn_bias, 'h i j -> 1 h i j').expand(batch, heads, -1, -1)
+
+            # if mask given, the mask would already contain the causal mask from above logic
+            # otherwise, if no mask given but still causal, mask out alibi positional bias to a large negative number
+
+            mask_value = -torch.finfo(q.dtype).max
+
+            if exists(mask):
+                attn_bias = attn_bias.masked_fill(~mask, mask_value // 2)
+            elif causal:
+                causal_mask = self.create_causal_mask(q_len, k_len, device = device)
+                attn_bias = attn_bias.masked_fill(causal_mask, mask_value // 2)
+                causal = False
+
+            # scaled_dot_product_attention handles attn_mask either as bool or additive bias
+            # make it an additive bias here
+
+            mask = attn_bias
+
+        # Check if there is a compatible device for flash attention
+
+        config = self.cuda_config if is_cuda else self.cpu_config
+
+        # pytorch 2.0 flash attn: q, k, v, mask, dropout, causal, softmax_scale
+        
+        # Legacy code...
+        # with torch.backends.cuda.sdp_kernel(enable_math=True, enable_mem_efficient=True):
+        # with sdpa_kernel([SDPBackend.MATH, SDPBackend.EFFICIENT_ATTENTION]):
+
+        # PyTorch 2.3-2.4 SDPA backend code...
+        with sdpa_kernel([SDPBackend.MATH, SDPBackend.EFFICIENT_ATTENTION, SDPBackend.FLASH_ATTENTION, SDPBackend.CUDNN_ATTENTION]):
+
+        # New PyTorch 2.5 SDPA backend code:
+        # with sdpa_kernel(SDPBackend.CUDNN_ATTENTION):
+            
+            out = F.scaled_dot_product_attention(
+                q, k, v,
+                attn_mask = mask,
+                dropout_p = self.dropout if self.training else 0., 
+                is_causal = causal
+            )
+
+        # for a row that is entirely masked out, should zero out the output of that row token
+
+        if exists(row_is_entirely_masked):
+            out = out.masked_fill(row_is_entirely_masked[..., None], 0.)
+
+        return out, Intermediates()
+
+    def forward(
+        self,
+        q, k, v,
+        mask = None,
+        attn_bias = None,
+        prev_attn = None
+    ):
+        """
+        einstein notation
+        b - batch
+        h - heads
+        n, i, j - sequence length (base sequence length, source, target)
+        d - feature dimension
+        """
+
+        n, heads, kv_heads, device = q.shape[-2], q.shape[1], k.shape[1], q.device
+
+        scale = default(self.scale, q.shape[-1] ** -0.5)
+
+        causal = self.causal
+
+        # handle kv cached decoding
+
+        if n == 1 and causal:
+            causal = False
+
+        # handle grouped multi-query attention
+
+        if kv_heads == 1:
+            k, v = map(lambda t: rearrange(t, 'b 1 n d -> b n d'), (k, v))
+        elif kv_heads < heads:
+            k, v = map(lambda t: repeat(t, 'b kvh n d -> b (r kvh) n d', r = heads // kv_heads), (k, v))
+
+        # handle zero kv, as means for allowing network to attend to nothing
+
+        if self.add_zero_kv:
+            k, v = map(lambda t: F.pad(t, (0, 0, 1, 0), value = 0.), (k, v))
+
+            if exists(mask):
+                mask = F.pad(mask, (1, 0), value = True)
+
+            if exists(attn_bias):
+                attn_bias = F.pad(attn_bias, (1, 0), value = 0.)
+
+        if self.flash:
+            assert not exists(prev_attn), 'residual attention not compatible with flash attention'
+            return self.flash_attn(q, k, v, mask = mask, attn_bias = attn_bias)
+
+        kv_einsum_eq = 'b j d' if k.ndim == 3 else 'b h j d'
+
+        dots = einsum(f'b h i d, {kv_einsum_eq} -> b h i j', q, k) * scale
+
+        if exists(prev_attn):
+            dots = dots + prev_attn
+
+        qk_similarities = dots.clone()
+
+        if self.talking_heads:
+            dots = self.pre_softmax_talking_heads(dots)
+
+        if exists(attn_bias):
+            dots = dots + attn_bias
+
+        i, j, dtype = *dots.shape[-2:], dots.dtype
+
+        mask_value = -torch.finfo(dots.dtype).max
+
+        if exists(self.sparse_topk) and self.sparse_topk < j:
+            top_values, _ = dots.topk(self.sparse_topk, dim = -1)
+            sparse_topk_mask = dots < top_values[..., -1:]
+            mask = (mask & sparse_topk_mask) if exists(mask) else sparse_topk_mask
+
+        if exists(mask):
+            dots = dots.masked_fill(~mask, mask_value)
+
+        if causal:
+            causal_mask = self.create_causal_mask(i, j, device = device)
+            dots = dots.masked_fill(causal_mask, mask_value)
+
+        pre_softmax_attn = dots.clone()
+
+        attn = self.attn_fn(dots, dim = -1)
+        attn = attn.type(dtype)
+
+        post_softmax_attn = attn.clone()
+
+        attn = self.attn_dropout(attn)
+
+        if self.talking_heads:
+            attn = self.post_softmax_talking_heads(attn)
+
+        out = einsum(f'b h i j, {kv_einsum_eq} -> b h i d', attn, v)
+
+        intermediates = Intermediates(
+            qk_similarities = qk_similarities,
+            pre_softmax_attn = pre_softmax_attn,
+            post_softmax_attn = post_softmax_attn
+        )
+
+        return out, intermediates
+
+#===================================================================================================================
+
+from math import ceil, log
+from typing import Optional, Union, Tuple, Callable
+
+import torch
+from torch import nn, Tensor
+from torch.nn import Module
+import torch.nn.functional as F
+
+from einops import rearrange, pack, unpack
+
+def exists(val):
+    return val is not None
+
+def default(val, d):
+    return val if exists(val) else d
+
+def identity(t, *args, **kwargs):
+    return t
+
+def cast_tuple(t, length = 1):
+    return t if isinstance(t, tuple) else (t,) * length
+
+def eval_decorator(fn):
+    def inner(self, *args, **kwargs):
+        was_training = self.training
+        self.eval()
+        out = fn(self, *args, **kwargs)
+        self.train(was_training)
+        return out
+    return inner
+
+# for variable lengthed prefixes
+
+def align_right(t, lens, pad_id = 0):
+    batch, seq_len, device, dtype = *t.shape, t.device, t.dtype
+
+    assert lens.ndim == 1 and lens.shape[0] == batch
+    assert lens.amax() <= seq_len
+
+    pad_lens = seq_len - lens
+    max_pad_len = pad_lens.amax()
+
+    batch_arange = torch.arange(batch, device = device, dtype = torch.long)[..., None]
+    prompt_len_arange = torch.arange(seq_len, device = device, dtype = torch.long)
+
+    t = F.pad(t, (max_pad_len, 0), value = 0)
+    offset = max_pad_len - pad_lens
+
+    aligned = t[batch_arange, prompt_len_arange + offset[..., None]]
+    return aligned
+
+# nucleus
+
+def top_p(logits, thres = 0.9):
+    sorted_logits, sorted_indices = torch.sort(logits, descending = True)
+    cum_probs = torch.cumsum(F.softmax(sorted_logits, dim = -1), dim = -1)
+
+    sorted_indices_to_remove = cum_probs > thres
+    sorted_indices_to_remove = F.pad(sorted_indices_to_remove, (1, -1), value = False)
+
+    sorted_logits[sorted_indices_to_remove] = float('-inf')
+    return sorted_logits.scatter(1, sorted_indices, sorted_logits)
+
+# topk
+
+def top_k(logits, frac_num_tokens = 0.1, k = None):
+    num_tokens = logits.shape[-1]
+
+    k = default(k, ceil(frac_num_tokens * num_tokens))
+    k = min(k, num_tokens)
+
+    val, ind = torch.topk(logits, k)
+    probs = torch.full_like(logits, float('-inf'))
+    probs.scatter_(1, ind, val)
+    return probs
+
+# top_a
+
+def top_a(logits, min_p_pow = 2.0, min_p_ratio = 0.02):
+    probs = F.softmax(logits, dim = -1)
+    max_probs = torch.amax(probs, dim = -1, keepdim = True)
+    limit = torch.pow(max_probs, min_p_pow) * min_p_ratio
+    return torch.where(probs < limit, float('-inf'), logits)
+
+# contrastive decoding function
+
+def contrastive_decode_fn(
+    expert_logits,
+    amateur_logits,
+    alpha = 0.1,
+    beta = 0.5
+):
+    """
+    Appendix A Algorithm 2
+    https://arxiv.org/abs/2309.09117
+    """
+
+    cutoff = log(alpha) + expert_logits.amax(dim = -1, keepdim = True)
+    diffs = (1 + beta) * expert_logits - beta * amateur_logits
+    contrastive_decode_logits = diffs.masked_fill(expert_logits < cutoff, -torch.finfo(expert_logits.dtype).max)
+    return contrastive_decode_logits
+
+# autoregressive wrapper class
+
+class AutoregressiveWrapper(Module):
+    def __init__(
+        self,
+        net,
+        ignore_index = -100,
+        pad_value = 0,
+        mask_prob = 0.,
+        add_attn_z_loss = False
+    ):
+        super().__init__()
+        self.pad_value = pad_value
+        self.ignore_index = ignore_index
+
+        self.net = net
+        self.max_seq_len = net.max_seq_len
+
+        # paper shows masking (MLM) in conjunction with autoregressive decoder-only training leads to big improvements https://arxiv.org/abs/2210.13432
+        assert mask_prob < 1.
+        self.mask_prob = mask_prob
+
+        # whether to add router z-loss
+        self.add_attn_z_loss = add_attn_z_loss
+
+    @torch.inference_mode()
+    @eval_decorator
+    def generate(
+        self,
+        prompts,
+        seq_len,
+        eos_token = None,
+        temperature = 1.,
+        prompt_lens: Optional[Tensor] = None,
+        filter_logits_fn: Callable = top_k,
+        restrict_to_max_seq_len = True,
+        amateur_model: Optional[Union[Module, Tuple[Module]]] = None,
+        filter_kwargs: dict = dict(),
+        contrastive_decode_kwargs: Union[dict, Tuple[dict]] = dict(
+            beta = 0.5,
+            alpha = 0.1
+        ),
+        cache_kv = True,
+        verbose=True,
+        return_prime=False,
+        **kwargs
+    ):
+        max_seq_len, device = self.max_seq_len, prompts.device
+
+        prompts, ps = pack([prompts], '* n')
+
+        b, t = prompts.shape
+
+        # handle variable lengthed prompts (prefixes)
+
+        seq_start_pos = None
+        if exists(prompt_lens):
+            prompts = align_right(prompts, prompt_lens, pad_id = self.pad_value)
+            seq_start_pos = t - prompt_lens
+
+        # output from which sampled tokens appended to
+
+        out = prompts
+
+        if verbose:
+          print("Generating sequence of max length:", seq_len)
+
+        # kv caches
+
+        cache = None
+
+        # if doing contrastive decoding, turn off filter automatically
+
+        if exists(amateur_model):
+            amateur_model = cast_tuple(amateur_model)
+            contrastive_decode_kwargs = cast_tuple(contrastive_decode_kwargs)
+
+            assert len(amateur_model) == len(contrastive_decode_kwargs)
+
+            amateur_caches = [None] * len(amateur_model)
+            filter_logits_fn = identity
+
+            for i, module in enumerate(amateur_model):
+                if isinstance(module, AutoregressiveWrapper):
+                    amateur_model[i] = module.net
+
+                module.eval()
+
+        # sampling up to seq_len
+
+        for sl in range(seq_len):
+
+            if restrict_to_max_seq_len:
+                x = out[:, -max_seq_len:]
+
+                if exists(cache):
+                    for inter in cache.attn_intermediates:
+                        inter.cached_kv = [t[..., -(max_seq_len - 1):, :] for t in inter.cached_kv]
+
+            logits, new_cache = self.net(
+                x,
+                return_intermediates = True,
+                cache = cache,
+                seq_start_pos = seq_start_pos,
+                **kwargs
+            )
+
+            if cache_kv and self.net.can_cache_kv:
+                cache = new_cache
+
+            logits = logits[:, -1]
+
+            # handle contrastive decoding, Li et al.
+            # https://arxiv.org/abs/2210.15097
+
+            if exists(amateur_model):
+                for i, (amateur, amateur_cache, amateur_contrastive_decode_kwargs) in enumerate(zip(amateur_model, amateur_caches, contrastive_decode_kwargs)):
+                    amateur_logits, next_amateur_cache = amateur(
+                        x,
+                        return_intermediates = True,
+                        cache = amateur_cache,
+                        seq_start_pos = seq_start_pos,
+                        **kwargs
+                    )
+
+                    amateur_logits = amateur_logits[:, -1]
+
+                    assert amateur_logits.shape == logits.shape, 'logits dimension are not the same between amateur and expert model'
+                    logits = contrastive_decode_fn(logits, amateur_logits, **amateur_contrastive_decode_kwargs)
+
+                    if cache_kv and amateur.can_cache_kv:
+                        amateur_caches[i] = next_amateur_cache
+
+            # filter by top_k, top_p (nucleus), top_a, or custom
+
+            filtered_logits = filter_logits_fn(logits, **filter_kwargs)
+
+            probs = F.softmax(filtered_logits / temperature, dim=-1)
+
+            sample = torch.multinomial(probs, 1)
+
+            out = torch.cat((out, sample), dim=-1)
+
+            if verbose:
+              if sl % 32 == 0:
+                print(sl, '/', seq_len)
+
+            if exists(eos_token):
+                is_eos_tokens = (out == eos_token)
+
+                if is_eos_tokens.any(dim = -1).all():
+                    # mask out everything after the eos tokens
+                    shifted_is_eos_tokens = F.pad(is_eos_tokens, (1, -1))
+                    mask = shifted_is_eos_tokens.float().cumsum(dim = -1) >= 1
+                    out = out.masked_fill(mask, self.pad_value)
+
+                    if verbose: 
+                      print('Model called the end of sequence at:', sl, '/', seq_len)
+
+                    break
+
+        if return_prime:
+          return out[:, :]
+        
+        else:
+          return out[:, t:]
+
+        # out, = unpack(out, ps, '* n')
+
+        # return out
+
+    def compute_accuracy(self, logits, labels): 
+        out = torch.argmax(logits, dim=-1) 
+        out = out.flatten() 
+        labels = labels.flatten() 
+
+        mask = (labels != self.ignore_index) # can also be self.pad_value (your choice)
+        out = out[mask] 
+        labels = labels[mask] 
+
+        num_right = (out == labels)
+        num_right = torch.sum(num_right).type(torch.float32)
+
+        acc = num_right / len(labels) 
+        return acc
+
+    def forward(self, x, **kwargs):
+        seq, ignore_index, add_attn_z_loss = x.shape[1], self.ignore_index, self.add_attn_z_loss
+
+        inp, target = x[:, :-1], x[:, 1:]
+        inp = torch.where(inp == ignore_index, self.pad_value, inp)
+
+        if self.mask_prob > 0.:
+            rand = torch.randn(inp.shape, device = x.device)
+            rand[:, 0] = -torch.finfo(rand.dtype).max # first token should not be masked out
+            num_mask = min(int(seq * self.mask_prob), seq - 1)
+            indices = rand.topk(num_mask, dim = -1).indices
+            mask = ~torch.zeros_like(inp).scatter(1, indices, 1.).bool()
+            kwargs.update(self_attn_kv_mask = mask)
+
+        logits, cache = self.net(
+            inp,
+            return_intermediates = True,
+            return_attn_z_loss = add_attn_z_loss,
+            **kwargs
+        )
+
+        acc = self.compute_accuracy(logits, target)
+
+        loss = F.cross_entropy(
+            rearrange(logits, 'b n c -> b c n'),
+            target,
+            ignore_index = ignore_index
+        )
+
+        if add_attn_z_loss:
+            loss = loss + cache.attn_z_loss
+
+        return loss, acc
+
+#===============================================================================
+
+import math
+from random import random
+
+import torch
+from torch import nn, einsum, Tensor
+import torch.nn.functional as F
+
+from functools import partial, wraps
+from inspect import isfunction
+from collections import namedtuple
+from dataclasses import dataclass
+from typing import List, Callable, Optional
+
+from einops import rearrange, repeat, reduce, pack, unpack
+from einops.layers.torch import Rearrange
+
+# constants
+
+DEFAULT_DIM_HEAD = 64
+
+@dataclass
+class LayerIntermediates:
+    hiddens: Optional[List[Tensor]] = None
+    attn_intermediates: Optional[List[Intermediates]] = None
+    layer_hiddens: Optional[List[Tensor]] = None
+    attn_z_loss: Optional[Tensor] = None
+    mems: Optional[Tensor] = None
+
+# helpers
+
+def exists(val):
+    return val is not None
+
+def default(val, d):
+    if exists(val):
+        return val
+    return d() if isfunction(d) else d
+
+def cast_tuple(val, depth):
+    return val if isinstance(val, tuple) else (val,) * depth
+
+def divisible_by(num, den):
+    return (num % den) == 0
+
+def maybe(fn):
+    @wraps(fn)
+    def inner(x, *args, **kwargs):
+        if not exists(x):
+            return x
+        return fn(x, *args, **kwargs)
+    return inner
+
+class always():
+    def __init__(self, val):
+        self.val = val
+    def __call__(self, *args, **kwargs):
+        return self.val
+
+class not_equals():
+    def __init__(self, val):
+        self.val = val
+    def __call__(self, x, *args, **kwargs):
+        return x != self.val
+
+class equals():
+    def __init__(self, val):
+        self.val = val
+    def __call__(self, x, *args, **kwargs):
+        return x == self.val
+
+def Sequential(*modules):
+    return nn.Sequential(*filter(exists, modules))
+
+# tensor helpers
+
+def max_neg_value(tensor):
+    return -torch.finfo(tensor.dtype).max
+
+def l2norm(t, groups = 1):
+    t = rearrange(t, '... (g d) -> ... g d', g = groups)
+    t = F.normalize(t, p = 2, dim = -1)
+    return rearrange(t, '... g d -> ... (g d)')
+
+def pad_at_dim(t, pad, dim = -1, value = 0.):
+    dims_from_right = (- dim - 1) if dim < 0 else (t.ndim - dim - 1)
+    zeros = ((0, 0) * dims_from_right)
+    return F.pad(t, (*zeros, *pad), value = value)
+
+def or_reduce(masks):
+    head, *body = masks
+    for rest in body:
+        head = head | rest
+    return head
+
+# auxiliary loss helpers
+
+def calc_z_loss(
+    pre_softmax_attns: List[Tensor],
+    mask = None,
+    weight = 1.
+):
+    # the same loss applied to the mixture of experts router logits in https://arxiv.org/abs/2202.08906
+    # in the paper, in a tiny footnote, they mention using it on attention logits with stabilizing effects
+    # also used in PaLM as one of the measures
+
+    lse = 0.
+
+    for attn in pre_softmax_attns:
+        lse = lse + attn.logsumexp(dim = -1)
+
+    loss = torch.square(lse)
+    loss = reduce(loss, 'b h n -> b n', 'sum')
+
+    if not exists(mask):
+        return loss.mean() * weight
+
+    loss = loss[mask].sum() / mask.sum().clamp(min = 1e-5)
+    return loss * weight
+
+# init helpers
+
+def init_zero_(layer):
+    nn.init.constant_(layer.weight, 0.)
+    if exists(layer.bias):
+        nn.init.constant_(layer.bias, 0.)
+
+# keyword argument helpers
+
+def pick_and_pop(keys, d):
+    values = list(map(lambda key: d.pop(key), keys))
+    return dict(zip(keys, values))
+
+def group_dict_by_key(cond, d):
+    return_val = [dict(),dict()]
+    for key in d.keys():
+        match = bool(cond(key))
+        ind = int(not match)
+        return_val[ind][key] = d[key]
+    return (*return_val,)
+
+def string_begins_with(prefix, str):
+    return str.startswith(prefix)
+
+def group_by_key_prefix(prefix, d):
+    return group_dict_by_key(partial(string_begins_with, prefix), d)
+
+def groupby_prefix_and_trim(prefix, d):
+    kwargs_with_prefix, kwargs = group_dict_by_key(partial(string_begins_with, prefix), d)
+    kwargs_without_prefix = dict(map(lambda x: (x[0][len(prefix):], x[1]), tuple(kwargs_with_prefix.items())))
+    return kwargs_without_prefix, kwargs
+
+# structured dropout, more effective than traditional attention dropouts
+
+def dropout_seq(seq, mask, dropout):
+    b, n, *_, device = *seq.shape, seq.device
+    logits = torch.randn(b, n, device = device)
+
+    if exists(mask):
+        mask_value = max_neg_value(logits)
+        logits = logits.masked_fill(~mask, mask_value)
+
+    keep_prob = 1. - dropout
+    num_keep = max(1,  int(keep_prob * n))
+    keep_indices = logits.topk(num_keep, dim = 1).indices
+
+    batch_indices = torch.arange(b, device = device)
+    batch_indices = rearrange(batch_indices, 'b -> b 1')
+
+    seq = seq[batch_indices, keep_indices]
+
+    if exists(mask):
+        seq_counts = mask.sum(dim = -1)
+        seq_keep_counts = torch.ceil(seq_counts * keep_prob).int()
+        keep_mask = torch.arange(num_keep, device = device) < rearrange(seq_keep_counts, 'b -> b 1')
+
+        mask = mask[batch_indices, keep_indices] & keep_mask
+
+    return seq, mask
+
+# activations
+
+class ReluSquared(nn.Module):
+    def forward(self, x):
+        return F.relu(x) ** 2
+
+# embedding
+
+class TokenEmbedding(nn.Module):
+    def __init__(self, dim, num_tokens, l2norm_embed = False):
+        super().__init__()
+        self.l2norm_embed = l2norm_embed
+        self.emb = nn.Embedding(num_tokens, dim)
+
+    def forward(self, x):
+        token_emb = self.emb(x)
+        return l2norm(token_emb) if self.l2norm_embed else token_emb
+
+# positional embeddings
+
+class AbsolutePositionalEmbedding(nn.Module):
+    def __init__(self, dim, max_seq_len, l2norm_embed = False):
+        super().__init__()
+        self.scale = dim ** -0.5 if not l2norm_embed else 1.
+        self.max_seq_len = max_seq_len
+        self.l2norm_embed = l2norm_embed
+        self.emb = nn.Embedding(max_seq_len, dim)
+
+    def forward(self, x, pos = None, seq_start_pos = None):
+        seq_len, device = x.shape[1], x.device
+        assert seq_len <= self.max_seq_len, f'you are passing in a sequence length of {seq_len} but your absolute positional embedding has a max sequence length of {self.max_seq_len}'
+
+        if not exists(pos):
+            pos = torch.arange(seq_len, device = device)
+
+        if exists(seq_start_pos):
+            pos = (pos - seq_start_pos[..., None]).clamp(min = 0)
+
+        pos_emb = self.emb(pos)
+        pos_emb = pos_emb * self.scale
+        return l2norm(pos_emb) if self.l2norm_embed else pos_emb
+
+class ScaledSinusoidalEmbedding(nn.Module):
+    def __init__(self, dim, theta = 10000):
+        super().__init__()
+        assert divisible_by(dim, 2)
+        self.scale = nn.Parameter(torch.ones(1) * dim ** -0.5)
+
+        half_dim = dim // 2
+        freq_seq = torch.arange(half_dim).float() / half_dim
+        inv_freq = theta ** -freq_seq
+        self.register_buffer('inv_freq', inv_freq, persistent = False)
+
+    def forward(self, x, pos = None, seq_start_pos = None):
+        seq_len, device = x.shape[1], x.device
+
+        if not exists(pos):
+            pos = torch.arange(seq_len, device = device)
+
+        if exists(seq_start_pos):
+            pos = pos - seq_start_pos[..., None]
+
+        emb = einsum('i, j -> i j', pos, self.inv_freq)
+        emb = torch.cat((emb.sin(), emb.cos()), dim = -1)
+        return emb * self.scale
+
+class RelativePositionBias(nn.Module):
+    def __init__(self, scale, causal = False, num_buckets = 32, max_distance = 128, heads = 8):
+        super().__init__()
+        self.scale = scale
+        self.causal = causal
+        self.num_buckets = num_buckets
+        self.max_distance = max_distance
+        self.relative_attention_bias = nn.Embedding(num_buckets, heads)
+
+    @staticmethod
+    def _relative_position_bucket(relative_position, causal = True, num_buckets = 32, max_distance = 128):
+        ret = 0
+        n = -relative_position
+        if not causal:
+            num_buckets //= 2
+            ret += (n < 0).long() * num_buckets
+            n = torch.abs(n)
+        else:
+            n = torch.max(n, torch.zeros_like(n))
+
+        max_exact = num_buckets // 2
+        is_small = n < max_exact
+
+        val_if_large = max_exact + (
+            torch.log(n.float() / max_exact) / math.log(max_distance / max_exact) * (num_buckets - max_exact)
+        ).long()
+        val_if_large = torch.min(val_if_large, torch.full_like(val_if_large, num_buckets - 1))
+
+        ret += torch.where(is_small, n, val_if_large)
+        return ret
+
+    @property
+    def device(self):
+        return next(self.parameters()).device
+
+    def forward(self, i, j):
+        device = self.device
+        q_pos = torch.arange(j - i, j, dtype = torch.long, device = device)
+        k_pos = torch.arange(j, dtype = torch.long, device = device)
+        rel_pos = k_pos[None, :] - q_pos[:, None]
+        rp_bucket = self._relative_position_bucket(rel_pos, causal = self.causal, num_buckets = self.num_buckets, max_distance = self.max_distance)
+        values = self.relative_attention_bias(rp_bucket)
+        bias = rearrange(values, 'i j h -> h i j')
+        return bias * self.scale
+
+class DynamicPositionBias(nn.Module):
+    def __init__(self, dim, *, heads, depth, log_distance = False, norm = False):
+        super().__init__()
+        assert depth >= 1, 'depth for dynamic position bias MLP must be greater or equal to 1'
+        self.log_distance = log_distance
+
+        self.mlp = nn.ModuleList([])
+
+        self.mlp.append(Sequential(
+            nn.Linear(1, dim),
+            nn.LayerNorm(dim) if norm else None,
+            nn.SiLU()
+        ))
+
+        for _ in range(depth - 1):
+            self.mlp.append(Sequential(
+                nn.Linear(dim, dim),
+                nn.LayerNorm(dim) if norm else None,
+                nn.SiLU()
+            ))
+
+        self.mlp.append(nn.Linear(dim, heads))
+
+    @property
+    def device(self):
+        return next(self.parameters()).device
+
+    def forward(self, i, j):
+        assert i == j
+        n, device = j, self.device
+
+        # get the (n x n) matrix of distances
+        seq_arange = torch.arange(n, device = device)
+        context_arange = torch.arange(n, device = device)
+        indices = rearrange(seq_arange, 'i -> i 1') - rearrange(context_arange, 'j -> 1 j')
+        indices += (n - 1)
+
+        # input to continuous positions MLP
+        pos = torch.arange(-n + 1, n, device = device).float()
+        pos = rearrange(pos, '... -> ... 1')
+
+        if self.log_distance:
+            pos = torch.sign(pos) * torch.log(pos.abs() + 1)  # log of distance is sign(rel_pos) * log(abs(rel_pos) + 1)
+
+        for layer in self.mlp:
+            pos = layer(pos)
+
+        # get position biases        
+        bias = pos[indices]
+        bias = rearrange(bias, 'i j h -> h i j')
+        return bias
+
+class AlibiPositionalBias(nn.Module):
+    def __init__(self, heads, total_heads, **kwargs):
+        super().__init__()
+        self.heads = heads
+        self.total_heads = total_heads
+
+        slopes = Tensor(self._get_slopes(heads))
+        slopes = rearrange(slopes, 'h -> h 1 1')
+        self.register_buffer('slopes', slopes, persistent = False)
+        self.register_buffer('bias', None, persistent = False)
+    
+    def get_bias(self, i, j, device):
+        i_arange = torch.arange(j - i, j, device = device)
+        j_arange = torch.arange(j, device = device)
+        bias = -torch.abs(rearrange(j_arange, 'j -> 1 1 j') - rearrange(i_arange, 'i -> 1 i 1'))
+        return bias
+
+    @staticmethod
+    def _get_slopes(heads):
+        def get_slopes_power_of_2(n):
+            start = (2**(-2**-(math.log2(n)-3)))
+            ratio = start
+            return [start*ratio**i for i in range(n)]
+
+        if math.log2(heads).is_integer():
+            return get_slopes_power_of_2(heads)
+
+        closest_power_of_2 = 2 ** math.floor(math.log2(heads))
+        return get_slopes_power_of_2(closest_power_of_2) + get_slopes_power_of_2(2 * closest_power_of_2)[0::2][:heads-closest_power_of_2]
+
+    @property
+    def device(self):
+        return next(self.buffers()).device
+
+    def forward(self, i, j):
+        h, device = self.total_heads, self.device
+
+        if exists(self.bias) and self.bias.shape[-1] >= j and self.bias.shape[-2] >= i:
+            return self.bias[..., -i:, -j:]
+
+        bias = self.get_bias(i, j, device)
+        bias = bias * self.slopes
+
+        num_heads_unalibied = h - bias.shape[0]
+        bias = pad_at_dim(bias, (0, num_heads_unalibied), dim = 0)
+        self.register_buffer('bias', bias, persistent = False)
+
+        return self.bias
+
+class RotaryEmbedding(nn.Module):
+    def __init__(
+        self,
+        dim,
+        use_xpos = False,
+        scale_base = 512,
+        interpolation_factor = 1.,
+        base = 10000,
+        base_rescale_factor = 1.
+    ):
+        super().__init__()
+        # proposed by reddit user bloc97, to rescale rotary embeddings to longer sequence length without fine-tuning
+        # has some connection to NTK literature
+        # https://www.reddit.com/r/LocalLLaMA/comments/14lz7j5/ntkaware_scaled_rope_allows_llama_models_to_have/
+        base *= base_rescale_factor ** (dim / (dim - 2))
+
+        inv_freq = 1. / (base ** (torch.arange(0, dim, 2).float() / dim))
+        self.register_buffer('inv_freq', inv_freq)
+
+        assert interpolation_factor >= 1.
+        self.interpolation_factor = interpolation_factor
+
+        if not use_xpos:
+            self.register_buffer('scale', None)
+            return
+
+        scale = (torch.arange(0, dim, 2) + 0.4 * dim) / (1.4 * dim)
+
+        self.scale_base = scale_base
+        self.register_buffer('scale', scale)
+
+    def forward(self, seq_len):
+        device = self.inv_freq.device
+        t = torch.arange(seq_len, device = device).type_as(self.inv_freq)
+
+        t = t / self.interpolation_factor
+
+        freqs = torch.einsum('i , j -> i j', t, self.inv_freq)
+        freqs = torch.cat((freqs, freqs), dim = -1)
+
+        if not exists(self.scale):
+            return freqs, 1.
+
+        power = (torch.arange(seq_len, device = device) - (seq_len // 2)) / self.scale_base
+        scale = self.scale ** rearrange(power, 'n -> n 1')
+        scale = torch.cat((scale, scale), dim = -1)
+
+        return freqs, scale
+
+
+def rotate_half(x):
+    x = rearrange(x, '... (j d) -> ... j d', j = 2)
+    x1, x2 = x.unbind(dim = -2)
+    return torch.cat((-x2, x1), dim = -1)
+
+def apply_rotary_pos_emb(t, freqs, scale = 1):
+    rot_dim, seq_len = freqs.shape[-1], t.shape[-2]
+    freqs = freqs[-seq_len:, :]
+
+    if t.ndim == 4 and freqs.ndim == 3:
+        freqs = rearrange(freqs, 'b n d -> b 1 n d')
+
+    # partial rotary embeddings, Wang et al. GPT-J
+    t, t_unrotated = t[..., :rot_dim], t[..., rot_dim:]
+    t = (t * freqs.cos() * scale) + (rotate_half(t) * freqs.sin() * scale)
+    return torch.cat((t, t_unrotated), dim = -1)
+
+# norms
+
+class Scale(nn.Module):
+    def __init__(self, value, fn):
+        super().__init__()
+        self.value = value
+        self.fn = fn
+
+    def forward(self, x, **kwargs):
+        out = self.fn(x, **kwargs)
+        scale_fn = lambda t: t * self.value
+
+        if not isinstance(out, tuple):
+            return scale_fn(out)
+
+        return (scale_fn(out[0]), *out[1:])
+
+class ScaleNorm(nn.Module):
+    def __init__(self, dim, eps = 1e-5):
+        super().__init__()
+        self.eps = eps
+        self.g = nn.Parameter(torch.ones(1) * (dim ** -0.5))
+
+    def forward(self, x):
+        norm = torch.norm(x, dim = -1, keepdim = True)
+        return x / norm.clamp(min = self.eps) * self.g
+
+class RMSNorm(nn.Module):
+    def __init__(self, dim):
+        super().__init__()
+        self.scale = dim ** 0.5
+        self.g = nn.Parameter(torch.ones(dim))
+
+    def forward(self, x):
+        return F.normalize(x, dim = -1) * self.scale * self.g
+
+class SimpleRMSNorm(nn.Module):
+    def __init__(self, dim):
+        super().__init__()
+        self.scale = dim ** 0.5
+
+    def forward(self, x):
+        return F.normalize(x, dim = -1) * self.scale
+
+# residual and residual gates
+
+class Residual(nn.Module):
+    def __init__(self, dim, scale_residual = False, scale_residual_constant = 1.):
+        super().__init__()
+        self.residual_scale = nn.Parameter(torch.ones(dim)) if scale_residual else None
+        self.scale_residual_constant = scale_residual_constant
+
+    def forward(self, x, residual):
+        if exists(self.residual_scale):
+            residual = residual * self.residual_scale
+
+        if self.scale_residual_constant != 1:
+            residual = residual * self.scale_residual_constant
+
+        return x + residual
+
+class GRUGating(nn.Module):
+    def __init__(self, dim, scale_residual = False, **kwargs):
+        super().__init__()
+        self.gru = nn.GRUCell(dim, dim)
+        self.residual_scale = nn.Parameter(torch.ones(dim)) if scale_residual else None
+
+    def forward(self, x, residual):
+        if exists(self.residual_scale):
+            residual = residual * self.residual_scale
+
+        gated_output = self.gru(
+            rearrange(x, 'b n d -> (b n) d'),
+            rearrange(residual, 'b n d -> (b n) d')
+        )
+
+        return gated_output.reshape_as(x)
+
+# token shifting
+
+def shift(t, amount, mask = None):
+    if amount == 0:
+        return t
+    else:
+        amount = min(amount, t.shape[1])
+
+    if exists(mask):
+        t = t.masked_fill(~mask[..., None], 0.)
+
+    return pad_at_dim(t, (amount, -amount), dim = - 2, value = 0.)
+
+class ShiftTokens(nn.Module):
+    def __init__(self, shifts, fn):
+        super().__init__()
+        self.fn = fn
+        self.shifts = tuple(shifts)
+
+    def forward(self, x, **kwargs):
+        mask = kwargs.get('mask', None)
+        shifts = self.shifts
+        segments = len(shifts)
+        feats_per_shift = x.shape[-1] // segments
+        splitted = x.split(feats_per_shift, dim = -1)
+        segments_to_shift, rest = splitted[:segments], splitted[segments:]
+        segments_to_shift = list(map(lambda args: shift(*args, mask = mask), zip(segments_to_shift, shifts)))
+        x = torch.cat((*segments_to_shift, *rest), dim = -1)
+        return self.fn(x, **kwargs)
+
+# feedforward
+
+class GLU(nn.Module):
+    def __init__(
+        self,
+        dim_in,
+        dim_out,
+        activation: Callable,
+        mult_bias = False
+    ):
+        super().__init__()
+        self.act = activation
+        self.proj = nn.Linear(dim_in, dim_out * 2)
+        self.mult_bias = nn.Parameter(torch.ones(dim_out)) if mult_bias else 1.
+
+    def forward(self, x):
+        x, gate = self.proj(x).chunk(2, dim = -1)
+        return x * self.act(gate) * self.mult_bias
+
+class FeedForward(nn.Module):
+    def __init__(
+        self,
+        dim,
+        dim_out = None,
+        mult = 4,
+        glu = False,
+        glu_mult_bias = False,
+        swish = False,
+        relu_squared = False,
+        post_act_ln = False,
+        dropout = 0.,
+        no_bias = False,
+        zero_init_output = False
+    ):
+        super().__init__()
+        inner_dim = int(dim * mult)
+        dim_out = default(dim_out, dim)
+
+        if relu_squared:
+            activation = ReluSquared()
+        elif swish:
+            activation = nn.SiLU()
+        else:
+            activation = nn.GELU()
+
+        if glu:
+            project_in = GLU(dim, inner_dim, activation, mult_bias = glu_mult_bias)
+        else:
+            project_in = nn.Sequential(
+                nn.Linear(dim, inner_dim, bias = not no_bias),
+                activation
+            )
+
+        self.ff = Sequential(
+            project_in,
+            nn.LayerNorm(inner_dim) if post_act_ln else None,
+            nn.Dropout(dropout),
+            nn.Linear(inner_dim, dim_out, bias = not no_bias)
+        )
+
+        # init last linear layer to 0
+        if zero_init_output:
+            init_zero_(self.ff[-1])
+
+    def forward(self, x):
+        return self.ff(x)
+
+# attention. it is all we need
+
+class Attention(nn.Module):
+    def __init__(
+        self,
+        dim,
+        dim_head = DEFAULT_DIM_HEAD,
+        heads = 8,
+        causal = False,
+        flash = False,
+        talking_heads = False,
+        head_scale = False,
+        sparse_topk = None,
+        num_mem_kv = 0,
+        dropout = 0.,
+        on_attn = False,
+        gate_value_heads = False,
+        gate_values = False,
+        zero_init_output = False,
+        max_attend_past = None,
+        qk_norm = False,
+        qk_norm_groups = 1,
+        qk_norm_scale = 10,
+        qk_norm_dim_scale = False,
+        one_kv_head = False,
+        kv_heads = None,
+        shared_kv = False,
+        value_dim_head = None,
+        tensor_product = False,      # https://arxiv.org/abs/2208.06061
+        add_zero_kv = False,         # same as add_zero_attn in pytorch
+        rotary_embed_values = False,
+        onnxable = False
+    ):
+        super().__init__()
+        self.scale = dim_head ** -0.5
+
+        self.heads = heads
+        self.causal = causal
+        self.max_attend_past = max_attend_past
+
+        assert not (exists(kv_heads) and one_kv_head), 'either attn_one_kv_head is set to True (in which case kv_heads is set to 1), or attn_kv_heads is set, but not both'
+
+        value_dim_head = default(value_dim_head, dim_head)
+        kv_heads = default(kv_heads, heads)
+
+        kv_heads = 1 if one_kv_head else kv_heads
+        assert divisible_by(heads, kv_heads)
+
+        self.kv_heads = kv_heads
+
+        q_dim = dim_head * heads
+        k_dim = dim_head * kv_heads
+        v_dim = value_dim_head * kv_heads
+        out_dim = value_dim_head * heads
+
+        self.to_q = nn.Linear(dim, q_dim, bias = False)
+        self.to_k = nn.Linear(dim, k_dim, bias = False)
+
+        # shared key / values, for further memory savings during inference
+        assert not (shared_kv and value_dim_head != dim_head), 'key and value head dimensions must be equal for shared key / values'
+        self.to_v = nn.Linear(dim, v_dim, bias = False) if not shared_kv else None
+
+        # relations projection from tp-attention
+        self.to_r = nn.Linear(dim, v_dim, bias = False) if tensor_product else None
+
+        # add GLU gating for aggregated values, from alphafold2
+        self.to_v_gate = None
+        if gate_values:
+            self.to_v_gate = nn.Linear(dim, out_dim)
+            nn.init.constant_(self.to_v_gate.weight, 0)
+            nn.init.constant_(self.to_v_gate.bias, 10)
+
+        # add per head gating of the output values, from 'Attend to nothing' paper
+        self.to_v_head_gate = None
+        if gate_value_heads:
+            self.to_v_head_gate = nn.Linear(dim, heads)
+            nn.init.constant_(self.to_v_head_gate.weight, 0)
+            nn.init.constant_(self.to_v_head_gate.bias, 10)
+
+        # cosine sim attention
+        self.qk_norm = qk_norm
+        self.qk_norm_groups = qk_norm_groups
+        self.qk_norm_scale = qk_norm_scale
+
+        # whether to use the rmsnorm (equivalent to cosine sim attention when scale is equal to 1) - https://arxiv.org/abs/2302.05442
+        self.qk_norm_dim_scale = qk_norm_dim_scale
+
+        self.qk_norm_q_scale = self.qk_norm_k_scale = 1
+        if qk_norm and qk_norm_dim_scale:
+            self.qk_norm_q_scale = nn.Parameter(torch.ones(heads, 1, dim_head))
+            self.qk_norm_k_scale = nn.Parameter(torch.ones(heads, 1, dim_head))
+
+        assert (not qk_norm) or divisible_by(dim_head, qk_norm_groups), 'dimension per attention head must be divisible by the qk norm groups'
+        assert not (qk_norm and (dim_head // qk_norm_groups) <= 2), 'the group dimension may be too small (2 was too small in my tests, but 4 still works, surprisingly)'
+
+        # attend class - includes core attention algorithm + talking heads
+
+        self.attend = Attend(
+            heads = heads,
+            causal = causal,
+            talking_heads = talking_heads,
+            dropout = dropout,
+            sparse_topk = sparse_topk,
+            qk_norm = qk_norm,
+            scale = qk_norm_scale if qk_norm else self.scale,
+            add_zero_kv = add_zero_kv,
+            flash = flash,
+            onnxable = onnxable
+        )
+
+        # head scaling
+        self.head_scale = head_scale
+        if head_scale:
+            self.head_scale_params = nn.Parameter(torch.ones(1, heads, 1, 1))
+
+        # explicit topk sparse attention
+        self.sparse_topk = sparse_topk
+
+        # add memory key / values
+        self.num_mem_kv = num_mem_kv
+        if num_mem_kv > 0:
+            self.mem_k = nn.Parameter(torch.randn(heads, num_mem_kv, dim_head))
+            self.mem_v = nn.Parameter(torch.randn(heads, num_mem_kv, dim_head))
+
+        # attention on attention
+        self.attn_on_attn = on_attn
+        self.to_out = nn.Sequential(nn.Linear(out_dim, dim * 2, bias = False), nn.GLU()) if on_attn else nn.Linear(out_dim, dim, bias = False)
+
+        # whether to rotate positions into values, for absolute positions in addition to relative
+        self.rotary_embed_values = rotary_embed_values
+
+        # init output projection 0
+        if zero_init_output:
+            init_zero_(self.to_out)
+
+    def forward(
+        self,
+        x,
+        context = None,
+        mask = None,
+        context_mask = None,
+        attn_mask = None,
+        rel_pos = None,
+        rotary_pos_emb = None,
+        prev_attn = None,
+        mem = None,
+        return_intermediates = False,
+        cache: Optional[Intermediates] = None,
+    ):
+        b, n, _, h, kv_h, head_scale, device, has_context = *x.shape, self.heads, self.kv_heads, self.head_scale, x.device, exists(context)
+        kv_input = default(context, x)
+
+        q_input = x
+        k_input = kv_input
+        v_input = kv_input
+        r_input = x
+
+        if exists(mem):
+            k_input, mem_packed_shape = pack([mem, k_input], 'b * d')
+            v_input, _ = pack([mem, v_input], 'b * d')
+
+        q = self.to_q(q_input)
+        k = self.to_k(k_input)
+        v = self.to_v(v_input) if exists(self.to_v) else k
+        r = self.to_r(r_input) if exists(self.to_r) else None
+
+        q = rearrange(q, 'b n (h d) -> b h n d', h = h)
+
+        k, v, r = map(lambda t: maybe(rearrange)(t, 'b n (h d) -> b h n d', h = kv_h), (k, v, r))
+
+        if exists(cache) and not has_context:
+            ck, cv = cache.cached_kv
+
+            if exists(mem):
+                mk, k = unpack(k, mem_packed_shape, 'b h * d')
+                mv, v = unpack(v, mem_packed_shape, 'b h * d')
+
+            k = torch.cat((ck, k), dim = -2)
+            v = torch.cat((cv, v), dim = -2)
+
+            if exists(mem):
+                k = torch.cat((mk, k), dim = -2)
+                v = torch.cat((mv, v), dim = -2)
+
+        if return_intermediates:
+            mem_len = mem.shape[-2] if exists(mem) else 0
+            cached_kv = (k[..., mem_len:, :], v[..., mem_len:, :])
+
+        if self.qk_norm:
+            qk_l2norm = partial(l2norm, groups = self.qk_norm_groups)
+            q, k = map(qk_l2norm, (q, k))
+            scale = self.qk_norm_scale
+
+            q = q * self.qk_norm_q_scale
+            k = k * self.qk_norm_k_scale
+
+        if exists(rotary_pos_emb) and not has_context:
+            freqs, xpos_scale = rotary_pos_emb
+            q_xpos_scale, k_xpos_scale = (xpos_scale, xpos_scale ** -1.) if exists(xpos_scale) else (1., 1.)
+
+            q = apply_rotary_pos_emb(q, freqs, q_xpos_scale)
+            k = apply_rotary_pos_emb(k, freqs, k_xpos_scale)
+
+            if self.rotary_embed_values:
+                v = apply_rotary_pos_emb(v, freqs, k_xpos_scale)
+
+        input_mask = context_mask
+
+        if not exists(input_mask) and not has_context:
+            input_mask = mask
+
+        if self.num_mem_kv > 0:
+            mem_k, mem_v = map(lambda t: repeat(t, 'h n d -> b h n d', b = b), (self.mem_k, self.mem_v))
+
+            if self.qk_norm:
+                mem_k = l2norm(mem_k)
+                mem_k = mem_k * self.qk_norm_k_scale
+
+            k = torch.cat((mem_k, k), dim = -2)
+            v = torch.cat((mem_v, v), dim = -2)
+
+            if exists(input_mask):
+                input_mask = pad_at_dim(input_mask, (self.num_mem_kv, 0), dim = -1, value = True)
+
+        i, j = map(lambda t: t.shape[-2], (q, k))
+
+        # determine masking
+
+        mask_value = max_neg_value(q)
+        masks = []
+        final_attn_mask = None
+
+        if exists(input_mask):
+            input_mask = rearrange(input_mask, 'b j -> b 1 1 j')
+            masks.append(~input_mask)
+
+        if exists(attn_mask):
+            assert 2 <= attn_mask.ndim <= 4, 'attention mask must have greater than 2 dimensions but less than or equal to 4'
+            if attn_mask.ndim == 2:
+                attn_mask = rearrange(attn_mask, 'i j -> 1 1 i j')
+            elif attn_mask.ndim == 3:
+                attn_mask = rearrange(attn_mask, 'h i j -> 1 h i j')
+            masks.append(~attn_mask)
+
+        if exists(self.max_attend_past):
+            range_q = torch.arange(j - i, j, device = device)
+            range_k = torch.arange(j, device = device)
+            dist = rearrange(range_q, 'i -> 1 1 i 1') - rearrange(range_k, 'j -> 1 1 1 j')
+            max_attend_past_mask = dist > self.max_attend_past
+            masks.append(max_attend_past_mask)
+
+        if len(masks) > 0:
+            final_attn_mask = ~or_reduce(masks)
+
+        # prepare relative positional bias, if needed
+
+        attn_bias = None
+        if exists(rel_pos):
+            attn_bias = rel_pos(i, j)
+
+        # attention is all we need
+
+        out, intermediates = self.attend(
+            q, k, v,
+            mask = final_attn_mask,
+            attn_bias = attn_bias,
+            prev_attn = prev_attn
+        )
+
+        # https://arxiv.org/abs/2208.06061 proposes to add a residual for better gradients
+
+        if exists(r):
+            out = out * r + out
+
+        # normformer scaling of heads
+
+        if head_scale:
+            out = out * self.head_scale_params
+
+        # per head gating, from https://arxiv.org/abs/2306.12929
+
+        if exists(self.to_v_head_gate):
+            head_gate = self.to_v_head_gate(x)
+            out = out * rearrange(head_gate, 'b n h -> b h n 1').sigmoid()
+
+        # merge heads
+
+        out = rearrange(out, 'b h n d -> b n (h d)')
+
+        # alphafold2 styled gating of the values
+
+        if exists(self.to_v_gate):
+            gates = self.to_v_gate(x)
+            out = out * gates.sigmoid()
+
+        # combine the heads
+
+        out = self.to_out(out)
+
+        if exists(mask):
+            mask = rearrange(mask, 'b n -> b n 1')
+            out = out.masked_fill(~mask, 0.)
+
+        if not return_intermediates:
+            return out
+
+        intermediates.cached_kv = cached_kv
+
+        return out, intermediates
+
+class AttentionLayers(nn.Module):
+    def __init__(
+        self,
+        dim,
+        depth,
+        heads = 8,
+        causal = False,
+        cross_attend = False,
+        only_cross = False,
+        use_scalenorm = False,
+        use_rmsnorm = False,
+        use_simple_rmsnorm = False,
+        alibi_pos_bias = False,
+        alibi_num_heads = None,
+        rel_pos_bias = False,
+        rel_pos_num_buckets = 32,
+        rel_pos_max_distance = 128,
+        dynamic_pos_bias = False,
+        dynamic_pos_bias_log_distance = False,
+        dynamic_pos_bias_mlp_depth = 2,
+        dynamic_pos_bias_norm = False,
+        rotary_pos_emb = False,
+        rotary_emb_dim = None,
+        rotary_xpos = False,
+        rotary_interpolation_factor = 1.,
+        rotary_xpos_scale_base = 512,
+        rotary_base_rescale_factor = 1.,
+        custom_layers = None,
+        sandwich_coef = None,
+        par_ratio = None,
+        weight_tie_layers = False,   # Albert - https://arxiv.org/abs/1909.11942
+        layers_execute_order = None, # generalizes weight tying, can do arbitrary layer execution orders
+        residual_attn = False,
+        cross_residual_attn = False,
+        macaron = False,
+        pre_norm = True,
+        pre_norm_has_final_norm = True,
+        gate_residual = False,
+        scale_residual = False,
+        scale_residual_constant = 1.,
+        shift_tokens = 0,
+        sandwich_norm = False,
+        resi_dual = False,
+        resi_dual_scale = 1.,
+        zero_init_branch_output = False,
+        layer_dropout = 0.,
+        cross_attn_tokens_dropout = 0.,
+        **kwargs
+    ):
+        super().__init__()
+        rotary_pos_emb = rotary_pos_emb or rotary_xpos
+
+        ff_kwargs, kwargs = groupby_prefix_and_trim('ff_', kwargs)
+        attn_kwargs, kwargs = groupby_prefix_and_trim('attn_', kwargs)
+
+        dim_head = attn_kwargs.get('dim_head', DEFAULT_DIM_HEAD)
+
+        self.dim = dim
+        self.depth = depth
+        self.causal = causal
+        self.layers = nn.ModuleList([])
+
+        self.has_pos_emb = rel_pos_bias or rotary_pos_emb
+
+        rotary_emb_dim = max(default(rotary_emb_dim, dim_head // 2), 32)
+
+        assert not (rotary_xpos and not causal), 'rotary xpos is not compatible with bidirectional attention'
+        self.rotary_pos_emb = RotaryEmbedding(rotary_emb_dim, use_xpos = rotary_xpos, scale_base = rotary_xpos_scale_base, interpolation_factor = rotary_interpolation_factor, base_rescale_factor = rotary_base_rescale_factor) if rotary_pos_emb else None
+
+        assert not (alibi_pos_bias and rel_pos_bias), 'you can only choose Alibi positional bias or T5 relative positional bias, not both'
+        assert rel_pos_num_buckets <= rel_pos_max_distance, 'number of relative position buckets must be less than the relative position max distance'
+
+        # relative positional bias
+
+        flash_attn = attn_kwargs.get('flash', False)
+        assert (int(rel_pos_bias) + int(dynamic_pos_bias) + int(alibi_pos_bias)) <= 1, 'you can only choose up to one of t5, alibi, or dynamic positional bias'
+
+        self.rel_pos = None
+        if rel_pos_bias:
+            assert not flash_attn, 'flash attention not compatible with t5 relative positional bias'
+            self.rel_pos = RelativePositionBias(scale = dim_head ** 0.5, causal = causal, heads = heads, num_buckets = rel_pos_num_buckets, max_distance = rel_pos_max_distance)
+        elif dynamic_pos_bias:
+            assert not flash_attn, 'flash attention not compatible with dynamic positional bias'
+            self.rel_pos = DynamicPositionBias(dim = dim // 4, heads = heads, log_distance = dynamic_pos_bias_log_distance, depth = dynamic_pos_bias_mlp_depth, norm = dynamic_pos_bias_norm)
+        elif alibi_pos_bias:
+            alibi_num_heads = default(alibi_num_heads, heads)
+            assert alibi_num_heads <= heads, 'number of ALiBi heads must be less than the total number of heads'
+            self.rel_pos = AlibiPositionalBias(heads = alibi_num_heads, total_heads = heads)
+
+        assert (int(sandwich_norm) + int(resi_dual)) <= 1, 'either sandwich norm or resiDual is selected, but not both'
+        assert not (not pre_norm and sandwich_norm), 'sandwich norm cannot be used when not using prenorm'
+
+        if resi_dual:
+            pre_norm = False
+
+        self.pre_norm = pre_norm
+        self.sandwich_norm = sandwich_norm
+
+        self.resi_dual = resi_dual
+        assert 0 < resi_dual_scale <= 1., 'resiDual prenorm residual must be scaled by a factor greater than 0 and less than or equal to 1.'
+        self.resi_dual_scale = resi_dual_scale
+
+        self.residual_attn = residual_attn
+        self.cross_residual_attn = cross_residual_attn
+        assert not (flash_attn and (residual_attn or cross_residual_attn)), 'flash attention is not compatible with residual attention'
+
+        self.cross_attend = cross_attend
+
+        assert (int(use_scalenorm) + int(use_rmsnorm) + int(use_simple_rmsnorm)) <= 1, 'you can only use either scalenorm, rmsnorm, or simple rmsnorm'
+
+        if use_scalenorm:
+            norm_class = ScaleNorm
+        elif use_rmsnorm:
+            norm_class = RMSNorm
+        elif use_simple_rmsnorm:
+            norm_class = SimpleRMSNorm
+        else:
+            norm_class = nn.LayerNorm
+
+        norm_fn = partial(norm_class, dim)
+
+        if cross_attend and not only_cross:
+            default_block = ('a', 'c', 'f')
+        elif cross_attend and only_cross:
+            default_block = ('c', 'f')
+        else:
+            default_block = ('a', 'f')
+
+        if macaron:
+            default_block = ('f',) + default_block
+
+        # zero init
+
+        if zero_init_branch_output:
+            attn_kwargs = {**attn_kwargs, 'zero_init_output':  True}
+            ff_kwargs = {**ff_kwargs, 'zero_init_output':  True}
+
+        # setup weight tying, which is a special case of `layer_execute_order`
+
+        assert not (weight_tie_layers and any([*map(exists, (custom_layers, par_ratio, sandwich_coef))]))
+
+        if weight_tie_layers:
+            assert not exists(layers_execute_order)
+            layers_execute_order = tuple(range(len(default_block))) * depth
+            depth = 1
+
+        # calculate layer block order
+
+        if exists(custom_layers):
+            layer_types = custom_layers
+        elif exists(par_ratio):
+            par_depth = depth * len(default_block)
+            assert 1 < par_ratio <= par_depth, 'par ratio out of range'
+            default_block = tuple(filter(not_equals('f'), default_block))
+            par_attn  = par_depth // par_ratio
+            depth_cut = par_depth * 2 // 3  # 2 / 3 attention layer cutoff suggested by PAR paper
+            par_width = (depth_cut + depth_cut // par_attn) // par_attn
+            assert len(default_block) <= par_width, 'default block is too large for par_ratio'
+            par_block = default_block + ('f',) * (par_width - len(default_block))
+            par_head = par_block * par_attn
+            layer_types = par_head + ('f',) * (par_depth - len(par_head))
+        elif exists(sandwich_coef):
+            assert sandwich_coef > 0 and sandwich_coef <= depth, 'sandwich coefficient should be less than the depth'
+            layer_types = ('a',) * sandwich_coef + default_block * (depth - sandwich_coef) + ('f',) * sandwich_coef
+        else:
+            layer_types = default_block * depth
+
+        self.layer_types = layer_types
+        self.layers_execute_order = default(layers_execute_order, tuple(range(len(layer_types))))
+
+        assert all([i < len(self.layer_types) for i in self.layers_execute_order])
+
+        self.num_attn_layers = len(list(filter(equals('a'), layer_types)))
+
+        # stochastic depth
+
+        self.layer_dropouts = cast_tuple(layer_dropout, len(layer_types))
+
+        # structured dropout for cross attending
+
+        self.cross_attn_tokens_dropout = cross_attn_tokens_dropout
+
+        # calculate token shifting
+
+        shift_tokens = cast_tuple(shift_tokens, len(layer_types))
+
+        # whether it has post norm
+
+        self.final_norm = norm_fn() if pre_norm or resi_dual else nn.Identity()
+
+        # iterate and construct layers
+
+        for ind, (layer_type, layer_shift_tokens) in enumerate(zip(self.layer_types, shift_tokens)):
+            is_last_layer = ind == (len(self.layer_types) - 1)
+
+            if layer_type == 'a':
+                layer = Attention(dim, heads = heads, causal = causal, **attn_kwargs)
+            elif layer_type == 'c':
+                layer = Attention(dim, heads = heads, **attn_kwargs)
+            elif layer_type == 'f':
+                layer = FeedForward(dim, **ff_kwargs)
+                layer = layer if not macaron else Scale(0.5, layer)
+            else:
+                raise Exception(f'invalid layer type {layer_type}')
+
+            if layer_shift_tokens > 0:
+                shift_range_upper = layer_shift_tokens + 1
+                shift_range_lower = -layer_shift_tokens if not causal else 0
+                layer = ShiftTokens(range(shift_range_lower, shift_range_upper), layer)
+
+            residual_fn = GRUGating if gate_residual else Residual
+            residual = residual_fn(dim, scale_residual = scale_residual, scale_residual_constant = scale_residual_constant)
+
+            pre_branch_norm = norm_fn() if pre_norm else None
+            post_branch_norm = norm_fn() if sandwich_norm else None
+            post_main_norm = norm_fn() if not pre_norm else None
+
+            norms = nn.ModuleList([
+                pre_branch_norm,
+                post_branch_norm,
+                post_main_norm
+            ])
+
+            self.layers.append(nn.ModuleList([
+                norms,
+                layer,
+                residual
+            ]))
+
+    def forward(
+        self,
+        x,
+        context = None,
+        mask = None,
+        context_mask = None,
+        attn_mask = None,
+        self_attn_kv_mask = None,
+        mems = None,
+        seq_start_pos: Optional[Tensor] = None,
+        cache: Optional[LayerIntermediates] = None,
+        cache_age = 1,
+        return_hiddens = False
+    ):
+        assert not (self.cross_attend ^ exists(context)), 'context must be passed in if cross_attend is set to True'
+
+        # initialize accums
+
+        hiddens = []
+        layer_hiddens = []
+        intermediates = []
+
+        prev_attn = None
+        prev_cross_attn = None
+
+        mems = mems.copy() if exists(mems) else [None] * self.num_attn_layers
+
+        # handle left padded sequences
+
+        if exists(seq_start_pos):
+            seq_arange = torch.arange(x.shape[-2], device = x.device, dtype = torch.long)
+            left_pad_mask = seq_arange >= seq_start_pos[..., None]
+
+            if exists(self_attn_kv_mask):
+                self_attn_kv_mask = self_attn_kv_mask & left_pad_mask
+            else:
+                self_attn_kv_mask = left_pad_mask
+
+        # rotary positions
+
+        rotary_pos_emb = None
+
+        if exists(self.rotary_pos_emb):
+            max_rotary_emb_length = max(list(map(lambda m: (m.shape[1] if exists(m) else 0) + x.shape[1], mems)))
+            rotary_pos_emb = self.rotary_pos_emb(max_rotary_emb_length)
+
+        # assume cached key / values
+
+        attn_cache = []
+
+        if exists(cache):
+            assert not self.training and self.causal and not any([*map(exists, (mask, attn_mask))])
+
+            if cache_age > 0:
+                x = x[:, -cache_age:] # for spec decoding, may be greater than 1
+
+            attn_cache = cache.attn_intermediates
+
+        iter_attn_cache = iter(attn_cache)
+
+        # outer residual - for resiDual paper
+
+        outer_residual = x * self.resi_dual_scale
+
+        # get layers to be executed
+
+        layer_variables = (
+            self.layer_types,
+            self.layers,
+            self.layer_dropouts
+        )
+
+        layer_variables = tuple(tuple(layer_variable[i] for i in self.layers_execute_order) for layer_variable in layer_variables)
+
+        # go through the attention and feedforward layers
+
+        for ind, (layer_type, (norm, block, residual_fn), layer_dropout) in enumerate(zip(*layer_variables)):
+            is_last = ind == (len(self.layers) - 1)
+
+            if self.training and layer_dropout > 0. and random() < layer_dropout:
+                continue
+
+            if layer_type == 'a':
+                if return_hiddens:
+                    hiddens.append(x)
+                layer_mem = mems.pop(0) if mems else None
+
+            if layer_type == 'c':
+                if self.training and self.cross_attn_tokens_dropout > 0.:
+                    context, context_mask = dropout_seq(context, context_mask, self.cross_attn_tokens_dropout)
+
+            inner_residual = x
+
+            if return_hiddens:
+                layer_hiddens.append(x)
+
+            pre_norm, post_branch_norm, post_main_norm = norm
+
+            if exists(pre_norm):
+                x = pre_norm(x)
+
+            if layer_type == 'a':
+                out, inter = block(x, mask = mask, context_mask = self_attn_kv_mask, attn_mask = attn_mask, rel_pos = self.rel_pos, rotary_pos_emb = rotary_pos_emb, prev_attn = prev_attn, cache = next(iter_attn_cache, None), mem = layer_mem, return_intermediates = True)
+            elif layer_type == 'c':
+                out, inter = block(x, context = context, mask = mask, context_mask = context_mask, prev_attn = prev_cross_attn, cache = next(iter_attn_cache, None), return_intermediates = True)
+            elif layer_type == 'f':
+                out = block(x)
+
+            if self.resi_dual:
+                outer_residual = outer_residual + out * self.resi_dual_scale
+
+            if exists(post_branch_norm):
+                out = post_branch_norm(out)
+
+            x = residual_fn(out, inner_residual)
+
+            if layer_type in ('a', 'c') and return_hiddens:
+                intermediates.append(inter)
+
+            if layer_type == 'a' and self.residual_attn:
+                prev_attn = inter.pre_softmax_attn
+            elif layer_type == 'c' and self.cross_residual_attn:
+                prev_cross_attn = inter.pre_softmax_attn
+
+            if exists(post_main_norm):
+                x = post_main_norm(x)
+
+        if return_hiddens:
+            layer_hiddens.append(x)
+
+        if self.resi_dual:
+            x = x + self.final_norm(outer_residual)
+        else:
+            x = self.final_norm(x)
+
+        if not return_hiddens:
+            return x
+
+        intermediates = LayerIntermediates(
+            hiddens = hiddens,
+            attn_intermediates = intermediates,
+            layer_hiddens = layer_hiddens
+        )
+
+        return x, intermediates
+
+class Encoder(AttentionLayers):
+    def __init__(self, **kwargs):
+        assert 'causal' not in kwargs, 'cannot set causality on encoder'
+        super().__init__(causal = False, **kwargs)
+
+class Decoder(AttentionLayers):
+    def __init__(self, **kwargs):
+        assert 'causal' not in kwargs, 'cannot set causality on decoder'
+        super().__init__(causal = True, **kwargs)
+
+class CrossAttender(AttentionLayers):
+    def __init__(self, **kwargs):
+        super().__init__(cross_attend = True, only_cross = True, **kwargs)
+
+class ViTransformerWrapper(nn.Module):
+    def __init__(
+        self,
+        *,
+        image_size,
+        patch_size,
+        attn_layers,
+        channels = 3,
+        num_classes = None,
+        post_emb_norm = False,
+        num_register_tokens = 0,
+        emb_dropout = 0.
+    ):
+        super().__init__()
+        assert isinstance(attn_layers, Encoder), 'attention layers must be an Encoder'
+        assert divisible_by(image_size, patch_size), 'image dimensions must be divisible by the patch size'
+        dim = attn_layers.dim
+        num_patches = (image_size // patch_size) ** 2
+        patch_dim = channels * patch_size ** 2
+
+        self.patch_size = patch_size
+
+        self.pos_embedding = nn.Parameter(torch.randn(1, num_patches, dim))
+
+        has_register_tokens = num_register_tokens > 0
+        self.has_register_tokens = has_register_tokens
+
+        if has_register_tokens:
+            self.register_tokens = nn.Parameter(torch.randn(num_register_tokens, dim))
+
+        self.patch_to_embedding = nn.Sequential(
+            nn.LayerNorm(patch_dim),
+            nn.Linear(patch_dim, dim),
+            nn.LayerNorm(dim)
+        )
+
+        self.post_emb_norm = nn.LayerNorm(dim) if post_emb_norm else nn.Identity()
+        self.dropout = nn.Dropout(emb_dropout)
+
+        self.attn_layers = attn_layers
+
+        self.mlp_head = nn.Linear(dim, num_classes) if exists(num_classes) else nn.Identity()
+
+    def forward(
+        self,
+        img,
+        return_embeddings = False
+    ):
+        b, p = img.shape[0], self.patch_size
+
+        x = rearrange(img, 'b c (h p1) (w p2) -> b (h w) (p1 p2 c)', p1 = p, p2 = p)
+        x = self.patch_to_embedding(x)
+        n = x.shape[1]
+
+        x = x + self.pos_embedding[:, :n]
+
+        x = self.post_emb_norm(x)
+        x = self.dropout(x)
+
+        if self.has_register_tokens:
+            r = repeat(self.register_tokens, 'n d -> b n d', b = b)
+            x, ps = pack((x, r), 'b * d')
+
+        x = self.attn_layers(x)
+
+        if self.has_register_tokens:
+            x, _ = unpack(x, ps, 'b * d')
+
+        if not exists(self.mlp_head) or return_embeddings:
+            return x
+
+        x = x.mean(dim = -2)
+        return self.mlp_head(x)
+
+class TransformerWrapper(nn.Module):
+    def __init__(
+        self,
+        *,
+        num_tokens,
+        max_seq_len,
+        attn_layers,
+        emb_dim = None,
+        max_mem_len = 0,
+        shift_mem_down = 0,
+        emb_dropout = 0.,
+        post_emb_norm = False,
+        num_memory_tokens = None,
+        memory_tokens_interspersed_every = None,
+        tie_embedding = False,
+        logits_dim = None,
+        use_abs_pos_emb = True,
+        scaled_sinu_pos_emb = False,
+        l2norm_embed = False,
+        emb_frac_gradient = 1., # GLM-130B and Cogview successfully used this, set at 0.1
+        attn_z_loss_weight = 1e-4,
+    ):
+        super().__init__()
+        assert isinstance(attn_layers, AttentionLayers), 'attention layers must be one of Encoder or Decoder'
+
+        dim = attn_layers.dim
+        emb_dim = default(emb_dim, dim)
+        self.emb_dim = emb_dim
+        self.num_tokens = num_tokens
+
+        self.max_seq_len = max_seq_len
+        self.max_mem_len = max_mem_len
+        self.shift_mem_down = shift_mem_down
+
+        self.l2norm_embed = l2norm_embed
+        self.token_emb = TokenEmbedding(emb_dim, num_tokens, l2norm_embed = l2norm_embed)
+
+        if not (use_abs_pos_emb and not attn_layers.has_pos_emb):
+            self.pos_emb = always(0)
+        elif scaled_sinu_pos_emb:
+            self.pos_emb = ScaledSinusoidalEmbedding(emb_dim)
+        else:
+            self.pos_emb = AbsolutePositionalEmbedding(emb_dim, max_seq_len, l2norm_embed = l2norm_embed)
+
+        self.emb_frac_gradient = emb_frac_gradient # fraction of the gradient that should go to the embedding, https://arxiv.org/abs/2105.13290
+
+        self.post_emb_norm = nn.LayerNorm(emb_dim) if post_emb_norm else nn.Identity()
+        self.emb_dropout = nn.Dropout(emb_dropout)
+
+        self.project_emb = nn.Linear(emb_dim, dim) if emb_dim != dim else nn.Identity()
+        self.attn_layers = attn_layers
+
+        self.init_()
+
+        logits_dim = default(logits_dim, num_tokens)
+        self.to_logits = nn.Linear(dim, logits_dim) if not tie_embedding else lambda t: t @ self.token_emb.emb.weight.t()
+
+        # memory tokens (like [cls]) from Memory Transformers paper
+
+        num_memory_tokens = default(num_memory_tokens, 0)
+        self.num_memory_tokens = num_memory_tokens
+        if num_memory_tokens > 0:
+            self.memory_tokens = nn.Parameter(torch.randn(num_memory_tokens, dim))
+
+        self.memory_tokens_interspersed_every = memory_tokens_interspersed_every
+
+        # whether can do cached kv decoding
+
+        self.can_cache_kv = self.num_memory_tokens == 0
+
+    def init_(self):
+        if self.l2norm_embed:
+            nn.init.normal_(self.token_emb.emb.weight, std = 1e-5)
+            if not isinstance(self.pos_emb, always):
+                nn.init.normal_(self.pos_emb.emb.weight, std = 1e-5)
+            return
+
+        nn.init.kaiming_normal_(self.token_emb.emb.weight)
+
+    def forward(
+        self,
+        x,
+        return_embeddings = False,
+        return_logits_and_embeddings = False,
+        return_intermediates = False,
+        mask = None,
+        return_mems = False,
+        return_attn = False,
+        mems = None,
+        pos = None,
+        prepend_embeds = None,
+        sum_embeds = None,
+        return_attn_z_loss = False,
+        attn_z_loss_weight = 1e-4,
+        seq_start_pos = None,
+        cache: Optional[LayerIntermediates] = None,
+        **kwargs
+    ):
+        b, n, device, num_mems, has_memory_tokens, emb_frac_gradient = *x.shape, x.device, self.num_memory_tokens, self.num_memory_tokens > 0, self.emb_frac_gradient
+        return_hiddens = return_mems | return_attn | return_intermediates | return_attn_z_loss
+
+        # absolute positional embedding
+
+        external_pos_emb = exists(pos) and pos.dtype != torch.long
+        pos_emb = self.pos_emb(x, pos = pos, seq_start_pos = seq_start_pos) if not external_pos_emb else pos
+        x = self.token_emb(x) + pos_emb
+
+        # for summing embeddings passed externally - needs this for self-conditioning in non-autoregressive training
+
+        if exists(sum_embeds):
+            x = x + sum_embeds
+
+        # post embedding norm, purportedly leads to greater stabilization
+
+        x = self.post_emb_norm(x)
+
+        # whether to append embeds, as in PaLI, for image embeddings
+
+        if exists(prepend_embeds):
+            prepend_seq, prepend_dim = prepend_embeds.shape[1:]
+            assert prepend_dim == x.shape[-1], 'prepended embeddings need to have same dimensions as text model dimensions'
+
+            x = torch.cat((prepend_embeds, x), dim = -2)
+
+        # whether to reduce the gradient going to the embedding, from cogview paper, corroborated by GLM-130B model
+
+        if emb_frac_gradient < 1:
+            assert emb_frac_gradient > 0
+            x = x * emb_frac_gradient + x.detach() * (1 - emb_frac_gradient)
+
+        # embedding dropout
+
+        x = self.emb_dropout(x)
+
+        x = self.project_emb(x)
+
+        if has_memory_tokens:
+            mem_every = self.memory_tokens_interspersed_every
+
+            if exists(mem_every):
+                assert mem_every > 0
+                assert isinstance(self.attn_layers, Decoder), 'only for decoder'
+                next_seq_len = math.ceil(n / mem_every) * mem_every
+
+                x = pad_at_dim(x, (0, next_seq_len - n), dim = -2, value = 0.)
+                x = rearrange(x, 'b (n m) d -> (b n) m d', m = mem_every)
+
+            mem = repeat(self.memory_tokens, 'n d -> b n d', b = x.shape[0])
+            x, mem_packed_shape = pack((mem, x), 'b * d')
+
+            # auto-handle masking after appending memory tokens
+            if not exists(mem_every) and exists(mask):
+                mask = pad_at_dim(mask, (num_mems, 0), dim = -1, value = True)
+
+            if exists(mem_every):
+                x = rearrange(x, '(b n) m d -> b (n m) d', b = b)
+
+        if self.shift_mem_down and exists(mems):
+            mems_l, mems_r = mems[:self.shift_mem_down], mems[self.shift_mem_down:]
+            mems = [*mems_r, *mems_l]
+
+        x, intermediates = self.attn_layers(x, mask = mask, mems = mems, cache = cache, return_hiddens = True, seq_start_pos = seq_start_pos, **kwargs)
+
+        if has_memory_tokens:
+            if exists(mem_every):
+                x = rearrange(x, 'b (n m) d -> (b n) m d', m = (mem_every + num_mems))
+
+            mem, x = unpack(x, mem_packed_shape, 'b * d')
+
+            if exists(mem_every):
+                x = rearrange(x, '(b n) m d -> b (n m) d', b = b)
+
+            x = x[:, :n]
+
+        if return_logits_and_embeddings:
+            out = (self.to_logits(x), x)
+        elif return_embeddings:
+            out = x
+        else:
+            out = self.to_logits(x)
+
+        if return_attn_z_loss:
+            pre_softmax_attns = list(map(lambda t: t.pre_softmax_attn, intermediates.attn_intermediates))
+            intermediates.attn_z_loss = calc_z_loss(pre_softmax_attns, weight = attn_z_loss_weight)
+            return_intermediates = True
+
+        if return_mems:
+            hiddens = intermediates.hiddens
+            new_mems = list(map(lambda pair: torch.cat(pair, dim = -2), zip(mems, hiddens))) if exists(mems) else hiddens
+            new_mems = list(map(lambda t: t[..., -self.max_mem_len:, :].detach(), new_mems))
+
+            if not return_intermediates:
+                return out, new_mems
+
+            intermediates.mems = new_mems
+
+        if return_intermediates:
+            return out, intermediates
+
+        if return_attn:
+            attn_maps = list(map(lambda t: t.post_softmax_attn, intermediates.attn_intermediates))
+            return out, attn_maps
+
+        return out
+
+class ContinuousTransformerWrapper(nn.Module):
+    def __init__(
+        self,
+        *,
+        max_seq_len,
+        attn_layers,
+        dim_in = None,
+        dim_out = None,
+        emb_dim = None,
+        max_mem_len = 0,
+        post_emb_norm = False,
+        emb_dropout = 0.,
+        use_abs_pos_emb = True,
+        scaled_sinu_pos_emb = False
+    ):
+        super().__init__()
+        assert isinstance(attn_layers, AttentionLayers), 'attention layers must be one of Encoder or Decoder'
+
+        dim = attn_layers.dim
+
+        self.max_seq_len = max_seq_len
+
+        self.max_mem_len = max_mem_len
+
+        if not (use_abs_pos_emb and not attn_layers.has_pos_emb):
+            self.pos_emb = always(0)
+        elif scaled_sinu_pos_emb:
+            self.pos_emb = ScaledSinusoidalEmbedding(dim)
+        else:
+            self.pos_emb = AbsolutePositionalEmbedding(dim, max_seq_len)
+
+        self.post_emb_norm = nn.LayerNorm(dim) if post_emb_norm else nn.Identity()
+        self.emb_dropout = nn.Dropout(emb_dropout)
+
+        self.project_in = nn.Linear(dim_in, dim) if exists(dim_in) else nn.Identity()
+
+        self.attn_layers = attn_layers
+
+        self.project_out = nn.Linear(dim, dim_out) if exists(dim_out) else nn.Identity()
+
+    def forward(
+        self,
+        x,
+        return_embeddings = False,
+        return_intermediates = False,
+        return_mems = False,
+        mask = None,
+        return_attn = False,
+        mems = None,
+        pos = None,
+        prepend_embeds = None,
+        **kwargs
+    ):
+        x = self.project_in(x)
+        x = x + self.pos_emb(x, pos = pos)
+
+        x = self.post_emb_norm(x)
+
+        # whether to append embeds, as in PaLI, for image embeddings
+
+        if exists(prepend_embeds):
+            _, prepend_dim = prepend_embeds.shape[1:]
+            assert prepend_dim == x.shape[-1], 'prepended embeddings need to have same dimensions as model dimensions'
+
+            x = torch.cat((prepend_embeds, x), dim = -2)
+
+        x = self.emb_dropout(x)
+
+        x, intermediates = self.attn_layers(x, mask = mask, mems = mems, return_hiddens = True, **kwargs)
+
+        out = self.project_out(x) if not return_embeddings else x
+
+        if return_intermediates:
+            return out, intermediates
+
+        if return_mems:
+            hiddens = intermediates.hiddens
+            new_mems = list(map(lambda t: t[..., -self.max_mem_len:, :].detach(), hiddens))
+            return out, new_mems
+
+        if return_attn:
+            attn_maps = list(map(lambda t: t.post_softmax_attn, intermediates.attn_intermediates))
+            return out, attn_maps
+
+        return out
+
+class XTransformer(nn.Module):
+    def __init__(
+        self,
+        *,
+        dim,
+        tie_token_emb = False,
+        ignore_index = -100,
+        pad_value = 0,
+        cross_attn_tokens_dropout = 0.,
+        **kwargs
+    ):
+        super().__init__()
+        enc_kwargs, kwargs = groupby_prefix_and_trim('enc_', kwargs)
+        dec_kwargs, kwargs = groupby_prefix_and_trim('dec_', kwargs)
+
+        assert 'dim' not in enc_kwargs and 'dim' not in dec_kwargs, 'dimension of either encoder or decoder must be set with `dim` keyword'
+        enc_transformer_kwargs = pick_and_pop(['num_tokens', 'max_seq_len'], enc_kwargs)
+        enc_transformer_kwargs['emb_dropout'] = enc_kwargs.pop('emb_dropout', 0)
+        enc_transformer_kwargs['num_memory_tokens'] = enc_kwargs.pop('num_memory_tokens', None)
+        enc_transformer_kwargs['scaled_sinu_pos_emb'] = enc_kwargs.pop('scaled_sinu_pos_emb', False)
+        enc_transformer_kwargs['use_abs_pos_emb'] = enc_kwargs.pop('use_abs_pos_emb', True)
+
+        dec_transformer_kwargs = pick_and_pop(['num_tokens', 'max_seq_len'], dec_kwargs)
+        dec_transformer_kwargs['emb_dropout'] = dec_kwargs.pop('emb_dropout', 0)
+        dec_transformer_kwargs['scaled_sinu_pos_emb'] = dec_kwargs.pop('scaled_sinu_pos_emb', False)
+        dec_transformer_kwargs['use_abs_pos_emb'] = dec_kwargs.pop('use_abs_pos_emb', True)
+
+        self.cross_attn_tokens_dropout = cross_attn_tokens_dropout  # how many tokens from the encoder to dropout when cross attending from decoder - seen in a couple papers, including Perceiver AR - this will also be very effective regularization when cross attending to very long memories
+
+        self.encoder = TransformerWrapper(
+            **enc_transformer_kwargs,
+            attn_layers = Encoder(dim = dim, **enc_kwargs)
+        )
+
+        self.decoder = TransformerWrapper(
+            **dec_transformer_kwargs,
+            attn_layers = Decoder(dim = dim, cross_attend = True, **dec_kwargs)
+        )
+
+        if tie_token_emb:
+            self.decoder.token_emb = self.encoder.token_emb
+
+        self.decoder = AutoregressiveWrapper(self.decoder, ignore_index=ignore_index, pad_value=pad_value)
+
+    @torch.no_grad()
+    def generate(self, seq_in, seq_out_start, seq_len, mask = None, attn_mask = None, **kwargs):
+        encodings = self.encoder(seq_in, mask = mask, attn_mask = attn_mask, return_embeddings = True)
+        return self.decoder.generate(seq_out_start, seq_len, context = encodings, context_mask = mask, **kwargs)
+
+    def forward(self, src, tgt, mask = None, attn_mask = None, src_prepend_embeds = None):
+
+        if exists(src_prepend_embeds) and exists(mask):
+            mask = pad_at_dim(mask, (src_prepend_embeds.shape[-2], 0), dim = -1, value = True)
+
+        enc = self.encoder(src, mask = mask, attn_mask = attn_mask, prepend_embeds = src_prepend_embeds, return_embeddings = True)
+
+        if self.training and self.cross_attn_tokens_dropout > 0:
+            enc, mask = dropout_seq(enc, mask, self.cross_attn_tokens_dropout)
+
+        out = self.decoder(tgt, context = enc, context_mask = mask)
+        return out