# Copyright (c) 2022, NVIDIA CORPORATION. All rights reserved.

# Parts of the code here are adapted from PyTorch
# repo: https://github.com/pytorch/pytorch

import math
import os
import warnings
from typing import Callable, Optional

import torch
import torch.nn.functional as F
import torch.nn.init as init
from torch.cuda.amp import custom_bwd, custom_fwd
from torch.nn.parameter import Parameter

from megatron.core.model_parallel_config import ModelParallelConfig
from megatron.core.parallel_state import (
    get_global_memory_buffer,
    get_tensor_model_parallel_group,
    get_tensor_model_parallel_rank,
    get_tensor_model_parallel_world_size,
)

from .mappings import (
    copy_to_tensor_model_parallel_region,
    gather_from_sequence_parallel_region,
    gather_from_tensor_model_parallel_region,
    reduce_from_tensor_model_parallel_region,
    reduce_scatter_to_sequence_parallel_region,
    scatter_to_tensor_model_parallel_region,
)
from .random import get_cuda_rng_tracker
from .utils import VocabUtility, divide, split_tensor_along_last_dim

_grad_accum_fusion_available = True
try:
    import fused_weight_gradient_mlp_cuda
except ImportError:
    _grad_accum_fusion_available = False

_MODEL_PARALLEL_ATTRIBUTE_DEFAULTS = {
    'tensor_model_parallel': False,
    'partition_dim': -1,
    'partition_stride': 1,
}


def param_is_not_tensor_parallel_duplicate(param):
    return (hasattr(param, 'tensor_model_parallel') and param.tensor_model_parallel) or (
        get_tensor_model_parallel_rank() == 0
    )


def set_tensor_model_parallel_attributes(tensor, is_parallel, dim, stride):
    # Make sure the attributes are not set.
    for attribute in _MODEL_PARALLEL_ATTRIBUTE_DEFAULTS:
        assert not hasattr(tensor, attribute)
    # Set the attributes.
    setattr(tensor, 'tensor_model_parallel', is_parallel)
    setattr(tensor, 'partition_dim', dim)
    setattr(tensor, 'partition_stride', stride)


def set_defaults_if_not_set_tensor_model_parallel_attributes(tensor):
    def maybe_set(attribute, value):
        if not hasattr(tensor, attribute):
            setattr(tensor, attribute, value)

    for attribute in _MODEL_PARALLEL_ATTRIBUTE_DEFAULTS:
        maybe_set(attribute, _MODEL_PARALLEL_ATTRIBUTE_DEFAULTS[attribute])


def copy_tensor_model_parallel_attributes(destination_tensor, source_tensor):
    def maybe_copy(attribute):
        if hasattr(source_tensor, attribute):
            setattr(destination_tensor, attribute, getattr(source_tensor, attribute))

    for attribute in _MODEL_PARALLEL_ATTRIBUTE_DEFAULTS:
        maybe_copy(attribute)


def _initialize_affine_weight_gpu(weight, init_method, partition_dim, stride=1):
    """Initialize affine weight for model parallel on GPU."""

    set_tensor_model_parallel_attributes(
        tensor=weight, is_parallel=True, dim=partition_dim, stride=stride
    )

    with get_cuda_rng_tracker().fork():
        init_method(weight)


def _initialize_affine_weight_cpu(
    weight,
    output_size,
    input_size,
    per_partition_size,
    partition_dim,
    init_method,
    stride=1,
    return_master_weight=False,
    *,
    params_dtype=torch.float32,
):
    """Initialize affine weight for model parallel.

    Build the master weight on all processes and scatter
    the relevant chunk."""

    set_tensor_model_parallel_attributes(
        tensor=weight, is_parallel=True, dim=partition_dim, stride=stride
    )

    # Initialize master weight
    master_weight = torch.empty(output_size, input_size, dtype=torch.float, requires_grad=False)
    init_method(master_weight)
    master_weight = master_weight.to(dtype=params_dtype)

    # Split and copy
    per_partition_per_stride_size = divide(per_partition_size, stride)
    weight_list = torch.split(master_weight, per_partition_per_stride_size, dim=partition_dim)
    rank = get_tensor_model_parallel_rank()
    world_size = get_tensor_model_parallel_world_size()
    my_weight_list = weight_list[rank::world_size]

    with torch.no_grad():
        torch.cat(my_weight_list, dim=partition_dim, out=weight)
    if return_master_weight:
        return master_weight
    return None


class VocabParallelEmbedding(torch.nn.Module):
    """Embedding parallelized in the vocabulary dimension.

    This is mainly adapted from torch.nn.Embedding and all the default
    values are kept.
    Arguments:
        num_embeddings: vocabulary size.
        embedding_dim: size of hidden state.

    Keyword Arguments:
        config: A megatron.core.ModelParallelConfig object
    """

    def __init__(
        self,
        num_embeddings: int,
        embedding_dim: int,
        *,
        init_method: Callable,
        config: ModelParallelConfig,
    ):
        super(VocabParallelEmbedding, self).__init__()
        # Keep the input dimensions.
        self.num_embeddings = num_embeddings
        self.embedding_dim = embedding_dim
        # Set the detauls for compatibility.
        self.padding_idx = None
        self.max_norm = None
        self.norm_type = 2.0
        self.scale_grad_by_freq = False
        self.sparse = False
        self._weight = None
        self.tensor_model_parallel_size = get_tensor_model_parallel_world_size()
        # Divide the weight matrix along the vocaburaly dimension.
        (
            self.vocab_start_index,
            self.vocab_end_index,
        ) = VocabUtility.vocab_range_from_global_vocab_size(
            self.num_embeddings, get_tensor_model_parallel_rank(), self.tensor_model_parallel_size
        )
        self.num_embeddings_per_partition = self.vocab_end_index - self.vocab_start_index

        # Allocate weights and initialize.
        if config.use_cpu_initialization:
            self.weight = Parameter(
                torch.empty(
                    self.num_embeddings_per_partition, self.embedding_dim, dtype=config.params_dtype
                )
            )
            if config.perform_initialization:
                _initialize_affine_weight_cpu(
                    self.weight,
                    self.num_embeddings,
                    self.embedding_dim,
                    self.num_embeddings_per_partition,
                    0,
                    init_method,
                    params_dtype=config.params_dtype,
                )
        else:
            self.weight = Parameter(
                torch.empty(
                    self.num_embeddings_per_partition,
                    self.embedding_dim,
                    device=torch.cuda.current_device(),
                    dtype=config.params_dtype,
                )
            )
            if config.perform_initialization:
                _initialize_affine_weight_gpu(self.weight, init_method, partition_dim=0, stride=1)

    def forward(self, input_):
        if self.tensor_model_parallel_size > 1:
            # Build the mask.
            input_mask = (input_ < self.vocab_start_index) | (input_ >= self.vocab_end_index)
            # Mask the input.
            masked_input = input_.clone() - self.vocab_start_index
            masked_input[input_mask] = 0
        else:
            masked_input = input_
            # Get the embeddings.
        output_parallel = F.embedding(
            masked_input,
            self.weight,
            self.padding_idx,
            self.max_norm,
            self.norm_type,
            self.scale_grad_by_freq,
            self.sparse,
        )
        # Mask the output embedding.
        if self.tensor_model_parallel_size > 1:
            output_parallel[input_mask, :] = 0.0
        # Reduce across all the model parallel GPUs.
        output = reduce_from_tensor_model_parallel_region(output_parallel)
        return output


class LinearWithFrozenWeight(torch.autograd.Function):
    """Linear operator that does not calculate gradient for weight.
    This op and LinearWithGradAccumulationAndAsyncCommunication performs 
    mathematically-identical forward and DGRAD. 
    
    Conceptually this op is the same as torch.nn.functional.linear with
    weight.requires_grad==False, but in experiments they are not identical 
    mathematically. """

    @staticmethod
    @custom_fwd
    def forward(
        ctx, input, weight, bias,
    ):
        ctx.save_for_backward(weight)
        output = torch.matmul(input, weight.t())
        if bias is not None:
            output = output + bias
        return output

    @staticmethod
    @custom_bwd
    def backward(ctx, grad_output):
        (weight,) = ctx.saved_tensors
        grad_input = grad_output.matmul(weight)
        return grad_input, None, None


def linear_with_frozen_weight(
    input: torch.Tensor,
    weight: torch.Tensor,
    bias: Optional[torch.Tensor],
    gradient_accumulation_fusion: bool,
    async_grad_allreduce: bool,
    sequence_parallel: bool,
) -> torch.Tensor:
    """Linear layer execution with weight.requires_grad == False.

    This function handles linear layers with weight frozen (untrainable). 
    In the forward, it only saves weight and does not save input activations.
    In the backward, it does not perform weight gradient calculation, or 
    weight gradient allreduce. 

    Arguments:

    input (torch.Tensor required): input like torch.nn.functional.linear

    weight (torch.Tensor required): weight like torch.nn.functional.linear

    bias (torch.Tensor optional): bias like torch.nn.functional.linear

    gradient_accumulation_fusion (bool required): dummy argument, used to 
    keep the API unified between all forward implementation functions.

    async_grad_allreduce (bool required): dummy argument, used to 
    keep the API unified between all forward implementation functions.

    sequence_parallel (bool required): Indicates that sequence
        parallelism is used and thus in the forward pass the input is
        all gathered, and the backward pass the input gradients are
        reduce scattered.
    """

    if sequence_parallel:
        input = gather_from_sequence_parallel_region(input, tensor_parallel_output_grad=True)
    else:
        input = input

    args = [
        input,
        weight,
        bias,
    ]

    return LinearWithFrozenWeight.apply(*args)


class LinearWithGradAccumulationAndAsyncCommunication(torch.autograd.Function):
    """See linear_with_grad_accumulation_and_async_allreduce"""

    @staticmethod
    @custom_fwd
    def forward(
        ctx,
        input,
        weight,
        bias,
        gradient_accumulation_fusion,
        async_grad_allreduce,
        sequence_parallel,
    ):
        ctx.save_for_backward(input, weight)
        ctx.use_bias = bias is not None
        ctx.gradient_accumulation_fusion = gradient_accumulation_fusion
        ctx.async_grad_allreduce = async_grad_allreduce
        ctx.sequence_parallel = sequence_parallel

        if sequence_parallel:
            world_size = get_tensor_model_parallel_world_size()
            dim_size = list(input.size())
            dim_size[0] = dim_size[0] * world_size

            all_gather_buffer = get_global_memory_buffer().get_tensor(dim_size, input.dtype, "mpu")
            torch.distributed._all_gather_base(
                all_gather_buffer, input, group=get_tensor_model_parallel_group()
            )
            total_input = all_gather_buffer
        else:
            total_input = input

        output = torch.matmul(total_input, weight.t())
        if bias is not None:
            output = output + bias
        return output

    @staticmethod
    @custom_bwd
    def backward(ctx, grad_output):
        input, weight = ctx.saved_tensors
        use_bias = ctx.use_bias

        if ctx.sequence_parallel:
            world_size = get_tensor_model_parallel_world_size()
            dim_size = list(input.size())
            dim_size[0] = dim_size[0] * world_size

            all_gather_buffer = get_global_memory_buffer().get_tensor(dim_size, input.dtype, "mpu")
            handle = torch.distributed._all_gather_base(
                all_gather_buffer, input, group=get_tensor_model_parallel_group(), async_op=True
            )

            # Here we rely on CUDA_DEVICE_MAX_CONNECTIONS=1 to ensure that the
            # gather is scheduled before the input gradient computation
            total_input = all_gather_buffer
        else:
            total_input = input
        grad_input = grad_output.matmul(weight)

        if ctx.sequence_parallel:
            handle.wait()

        # Doing gather + slicing during the NeMo forward pass can make this tensor
        # not be contiguous. PyTorch only checks if the tensor is contiguous, and only
        # clones it if it's not contiguous:
        # https://github.com/pytorch/pytorch/blob/c47cf9bc7f9e02f649ab4ed53fe4d35732c92ab6/torch/_refs/__init__.py#L2761
        grad_output = grad_output.contiguous()
        # Convert the tensor shapes to 2D for execution compatibility
        grad_output = grad_output.view(
            grad_output.shape[0] * grad_output.shape[1], grad_output.shape[2]
        )
        total_input = total_input.view(
            total_input.shape[0] * total_input.shape[1], total_input.shape[2]
        )

        if ctx.async_grad_allreduce:
            # Asynchronous all-reduce
            handle = torch.distributed.all_reduce(
                grad_input, group=get_tensor_model_parallel_group(), async_op=True
            )
            # Here we rely on CUDA_DEVICE_MAX_CONNECTIONS=1 to ensure that the
            # all-reduce is scheduled before the weight gradient computation

        if ctx.sequence_parallel:
            assert not ctx.async_grad_allreduce
            dim_size = list(input.size())
            sub_grad_input = torch.empty(
                dim_size, dtype=input.dtype, device=torch.cuda.current_device(), requires_grad=False
            )
            # reduce_scatter
            handle = torch.distributed._reduce_scatter_base(
                sub_grad_input, grad_input, group=get_tensor_model_parallel_group(), async_op=True
            )
            # Here we rely on CUDA_DEVICE_MAX_CONNECTIONS=1 to ensure that the
            # reduce scatter is scheduled before the weight gradient computation

        if ctx.gradient_accumulation_fusion:
            if weight.main_grad.dtype == torch.float32:
                fused_weight_gradient_mlp_cuda.wgrad_gemm_accum_fp32(
                    total_input, grad_output, weight.main_grad
                )
            elif weight.main_grad.dtype in (torch.float16, torch.bfloat16):
                fused_weight_gradient_mlp_cuda.wgrad_gemm_accum_fp16(
                    total_input, grad_output, weight.main_grad
                )
            else:
                raise RuntimeError("Unsupported gradient type for gradient accumulation fusion")

            if hasattr(weight, 'grad_added_to_main_grad'):
                # When overlap_grad_reduce is True, need to ensure that backward hooks
                # are all run on the main backprop thread to prevent deadlocks. Setup
                # dummy grad_weight tensor to prevent backward hooks from being run
                # in a background thread.
                grad_weight = torch.empty(
                    weight.main_grad.shape,
                    dtype=input.dtype,
                    device=torch.cuda.current_device(),
                    requires_grad=False,
                )
                weight.grad_added_to_main_grad = True
            else:
                grad_weight = None
        else:
            grad_weight = grad_output.t().matmul(total_input)
        grad_bias = grad_output.sum(dim=0) if use_bias else None

        if ctx.sequence_parallel:
            handle.wait()
            return sub_grad_input, grad_weight, grad_bias, None, None, None

        if ctx.async_grad_allreduce:
            handle.wait()

        return grad_input, grad_weight, grad_bias, None, None, None


def linear_with_grad_accumulation_and_async_allreduce(
    input: torch.Tensor,
    weight: torch.Tensor,
    bias: Optional[torch.Tensor],
    gradient_accumulation_fusion: bool,
    async_grad_allreduce: bool,
    sequence_parallel: bool,
) -> torch.Tensor:
    """Linear layer execution with asynchronous communication and
    gradient accumulation fusion in backprop.

    This has the option to accumulate the result of backprop
    calculation into an existing gradient buffer, preventing the need
    to do an additional addition kernel after the gradient
    calculation.

    Additionally, the tensor parallel all reduce of the input
    gradients can be done asynchronously with the calculation of
    the weight gradients.

    In the case of sequence parallelism, the reduce scatter of the
    input gradients is done asynchronously with the calcluation of the
    weight gradients.

    Use of this module requires that the environment variable
    CUDA_DEVICE_MAX_CONNECTIONS=1. There are a few collective
    operations, noted in the code, that should be scheduled before
    compute kernels to overlap the communication with the computation,
    which is necessary for a speedup but not for correctness so that
    ordering isn't imposed by the scheduler. Setting
    CUDA_DEVICE_MAX_CONNECTIONS=1 forces the kernels to be scheduled
    in the order they are called.

    Arguments:

    input (torch.Tensor required): input like torch.nn.functional.linear

    weight (torch.Tensor required): weight like torch.nn.functional.linear

    bias (torch.Tensor optional): bias like torch.nn.functional.linear

    gradient_accumulation_fusion (bool required): Perform the gradient
        accumulation fusion, requires the custom CUDA extension
        fused_weight_gradient_mlp_cuda module. To use
        gradient_accumulation_fusion you must install APEX with
        --cpp_ext and --cuda_ext. For example: "pip install
        --global-option=\"--cpp_ext\" --global-option=\"--cuda_ext .\"
        " Note that the extension requires CUDA>=11. Otherwise, you
        must turn off gradient accumulation fusion."

    async_grad_allreduce (bool required): Do the allreduce of input
        gradients asyncronously with the computation of weight
        gradients. If sequence_parallel is True, this must be
        False, as no all reduce is performed.

    sequence_parallel (bool required): Indicates that sequence
        parallelism is used and thus in the forward pass the input is
        all gathered, and the backward pass the input gradients are
        reduce scattered.
    """
    args = [
        input,
        weight,
        bias,
        gradient_accumulation_fusion,
        async_grad_allreduce,
        sequence_parallel,
    ]

    if not linear_with_grad_accumulation_and_async_allreduce.warned:
        if os.environ.get('CUDA_DEVICE_MAX_CONNECTIONS') != "1":
            if sequence_parallel:
                warnings.warn(
                    "When using sequence parallelism it is recommended to set the "
                    "environment variable CUDA_DEVICE_MAX_CONNECTIONS to 1 for "
                    "maximum speedup"
                )
                linear_with_grad_accumulation_and_async_allreduce.warned = True

            if async_grad_allreduce:
                warnings.warn(
                    "When using async grad allreduce it is recommended to set the "
                    "environment variable CUDA_DEVICE_MAX_CONNECTIONS to 1 for "
                    "maximum speedup"
                )
                linear_with_grad_accumulation_and_async_allreduce.warned = True

    return LinearWithGradAccumulationAndAsyncCommunication.apply(*args)


linear_with_grad_accumulation_and_async_allreduce.warned = False


class ColumnParallelLinear(torch.nn.Module):
    """Linear layer with column parallelism.

    The linear layer is defined as Y = XA + b. A is parallelized along
    its second dimension as A = [A_1, ..., A_p].

    Arguments:
        input_size: first dimension of matrix A.
        output_size: second dimension of matrix A.

    Keyword Arguments
        bias: If true, add bias
        gather_output: If true, call all-gather on output and make Y available
                       to all GPUs, otherwise, every GPU will have its output
                       which is Y_i = XA_i
        init_method: method to initialize weights. Note that bias is always set
                     to zero.
        stride: For the strided linear layers.
        keep_master_weight_for_test: This was added for testing and should be
                                     set to False. It returns the master weights
                                     used for initialization.
        skip_bias_add: If True, do not add the bias term, instead
                       return it to be added by the caller. This
                       enables performance optimations where bias can
                       be fused with other elementwise operations.

        skip_weight_param_allocation: If True, weight parameter is not allocated and must be passed
                                      as a keyword argument `weight` during the forward pass. Note
                                      that this does not affect bias, which will be allocated if
                                      bias is True. Defaults to False.

        config: ModelParallelConfig object

    """

    def __init__(
        self,
        input_size,
        output_size,
        *,
        config: ModelParallelConfig,
        init_method: Callable,
        bias=True,
        gather_output=False,
        stride=1,
        keep_master_weight_for_test=False,
        skip_bias_add=False,
        skip_weight_param_allocation: bool = False,
    ):
        super(ColumnParallelLinear, self).__init__()

        # Keep input parameters
        self.input_size = input_size
        self.output_size = output_size
        self.gather_output = gather_output
        # Divide the weight matrix along the last dimension.
        world_size = get_tensor_model_parallel_world_size()
        self.output_size_per_partition = divide(output_size, world_size)
        self.skip_bias_add = skip_bias_add
        self.config = config

        # Parameters.
        # Note: torch.nn.functional.linear performs XA^T + b and as a result
        # we allocate the transpose.
        # Initialize weight.
        if not skip_weight_param_allocation:
            if config.use_cpu_initialization:
                self.weight = Parameter(
                    torch.empty(
                        self.output_size_per_partition, self.input_size, dtype=config.params_dtype
                    )
                )
                if config.perform_initialization:
                    self.master_weight = _initialize_affine_weight_cpu(
                        self.weight,
                        self.output_size,
                        self.input_size,
                        self.output_size_per_partition,
                        0,
                        init_method,
                        stride=stride,
                        return_master_weight=keep_master_weight_for_test,
                    )
            else:
                self.weight = Parameter(
                    torch.empty(
                        self.output_size_per_partition,
                        self.input_size,
                        device=torch.cuda.current_device(),
                        dtype=config.params_dtype,
                    )
                )
                if config.perform_initialization:
                    _initialize_affine_weight_gpu(
                        self.weight, init_method, partition_dim=0, stride=stride
                    )
        else:
            self.weight = None

        if bias:
            if config.use_cpu_initialization:
                self.bias = Parameter(
                    torch.empty(self.output_size_per_partition, dtype=config.params_dtype)
                )
            else:
                self.bias = Parameter(
                    torch.empty(
                        self.output_size_per_partition,
                        device=torch.cuda.current_device(),
                        dtype=config.params_dtype,
                    )
                )
            set_tensor_model_parallel_attributes(self.bias, True, 0, stride)
            if config.perform_initialization:
                # Always initialize bias to zero.
                with torch.no_grad():
                    self.bias.zero_()
        else:
            self.register_parameter('bias', None)

        self.async_tensor_model_parallel_allreduce = (
            config.async_tensor_model_parallel_allreduce and world_size > 1
        )

        self.sequence_parallel = config.sequence_parallel
        if self.sequence_parallel and world_size <= 1:
            warnings.warn(
                f"`sequence_parallel` is set to `True`, but tensor model parallel size is {world_size}. "
                f"Disabling sequence parallel."
            )
            self.sequence_parallel = False

        if config.gradient_accumulation_fusion and not _grad_accum_fusion_available:
            raise RuntimeError(
                "ColumnParallelLinear was called with gradient_accumulation_fusion set "
                "to True but the custom CUDA extension fused_weight_gradient_mlp_cuda "
                "module is not found. To use gradient_accumulation_fusion you must "
                "install APEX with --cpp_ext and --cuda_ext. For example: "
                "pip install --global-option=\"--cpp_ext\" --global-option=\"--cuda_ext .\" "
                "Note that the extension requires CUDA>=11. Otherwise, you must turn off "
                "gradient accumulation fusion."
            )
        self.gradient_accumulation_fusion = config.gradient_accumulation_fusion

        if self.async_tensor_model_parallel_allreduce and self.sequence_parallel:
            raise RuntimeError(
                "`async_tensor_model_parallel_allreduce` and `sequence_parallel` "
                "cannot be enabled at the same time."
            )

        self._forward_impl = linear_with_grad_accumulation_and_async_allreduce

    def forward(self, input_: torch.Tensor, weight: Optional[torch.Tensor] = None):
        """Forward of ColumnParallelLinear

        Args:
            input_: 3D tensor whose order of dimension is [sequence, batch, hidden]

            weight (optional): weight tensor to use, compulsory when
                skip_weight_param_allocation is True.

        Returns:
            - output
            - bias

        """
        if weight is None:
            if self.weight is None:
                raise RuntimeError(
                    "weight was not supplied to ColumnParallelLinear forward pass "
                    "and skip_weight_param_allocation is True."
                )
            weight = self.weight
        else:
            # Check the weight passed in is the correct shape
            expected_shape = (self.output_size_per_partition, self.input_size)
            if weight.shape != expected_shape:
                raise RuntimeError(
                    f"supplied weight's shape is {tuple(weight.shape)}, "
                    f"not {expected_shape} as expected"
                )

        bias = self.bias if not self.skip_bias_add else None

        if self.async_tensor_model_parallel_allreduce or self.sequence_parallel:
            input_parallel = input_
        else:
            input_parallel = copy_to_tensor_model_parallel_region(input_)
        # Matrix multiply.
        if not weight.requires_grad:
            self._forward_impl = linear_with_frozen_weight
        else:
            self._forward_impl = linear_with_grad_accumulation_and_async_allreduce
        output_parallel = self._forward_impl(
            input=input_parallel,
            weight=weight,
            bias=bias,
            gradient_accumulation_fusion=self.gradient_accumulation_fusion,
            async_grad_allreduce=self.async_tensor_model_parallel_allreduce,
            sequence_parallel=self.sequence_parallel,
        )
        if self.gather_output:
            # All-gather across the partitions.
            assert not self.sequence_parallel
            output = gather_from_tensor_model_parallel_region(output_parallel)
        else:
            output = output_parallel
        output_bias = self.bias if self.skip_bias_add else None
        return output, output_bias


class RowParallelLinear(torch.nn.Module):
    """Linear layer with row parallelism.

    The linear layer is defined as Y = XA + b. A is parallelized along
    its first dimension and X along its second dimension as:
               -   -
              | A_1 |
              | .   |
          A = | .   |        X = [X_1, ..., X_p]
              | .   |
              | A_p |
               -   -
    Arguments:
        input_size: first dimension of matrix A.
        output_size: second dimension of matrix A.

    Keyword Arguments:
        bias: If true, add bias. Note that bias is not parallelized.
        input_is_parallel: If true, we assume that the input is already
                           split across the GPUs and we do not split
                           again.
        init_method: method to initialize weights. Note that bias is always set
                     to zero.
        stride: For the strided linear layers.
        keep_master_weight_for_test: This was added for testing and should be
                                     set to False. It returns the master weights
                                     used for initialization.
        skip_bias_add: If True, do not add the bias term, instead
                       return it to be added by the caller. This
                       enables performance optimations where bias can
                       be fused with other elementwise operations.
        config: ModelParallelConfig object

    """

    def __init__(
        self,
        input_size: int,
        output_size: int,
        *,
        config: ModelParallelConfig,
        init_method: Callable,
        bias: bool = True,
        input_is_parallel: bool = False,
        stride: int = 1,
        keep_master_weight_for_test: bool = False,
        skip_bias_add: bool = False,
    ):
        super(RowParallelLinear, self).__init__()

        # Keep input parameters
        self.input_size = input_size
        self.output_size = output_size
        self.input_is_parallel = input_is_parallel
        # Divide the weight matrix along the last dimension.
        world_size = get_tensor_model_parallel_world_size()
        self.input_size_per_partition = divide(input_size, world_size)
        self.skip_bias_add = skip_bias_add
        self.config = config
        self.gradient_accumulation_fusion = config.gradient_accumulation_fusion
        self.sequence_parallel = config.sequence_parallel
        if self.sequence_parallel and not self.input_is_parallel:
            raise RuntimeError("To enable `sequence_parallel`, `input_is_parallel` must be `True`")

        # Parameters.
        # Note: torch.nn.functional.linear performs XA^T + b and as a result
        # we allocate the transpose.
        # Initialize weight.
        if config.use_cpu_initialization:
            self.weight = Parameter(
                torch.empty(
                    self.output_size, self.input_size_per_partition, dtype=config.params_dtype
                )
            )
            if config.perform_initialization:
                self.master_weight = _initialize_affine_weight_cpu(
                    self.weight,
                    self.output_size,
                    self.input_size,
                    self.input_size_per_partition,
                    1,
                    init_method,
                    stride=stride,
                    return_master_weight=keep_master_weight_for_test,
                    params_dtype=config.params_dtype,
                )
        else:
            self.weight = Parameter(
                torch.empty(
                    self.output_size,
                    self.input_size_per_partition,
                    device=torch.cuda.current_device(),
                    dtype=config.params_dtype,
                )
            )
            if config.perform_initialization:
                _initialize_affine_weight_gpu(
                    self.weight, init_method, partition_dim=1, stride=stride
                )
        if bias:
            if config.use_cpu_initialization:
                self.bias = Parameter(torch.empty(self.output_size, dtype=config.params_dtype))
            else:
                self.bias = Parameter(
                    torch.empty(
                        self.output_size,
                        device=torch.cuda.current_device(),
                        dtype=config.params_dtype,
                    )
                )
            setattr(self.bias, 'sequence_parallel', self.sequence_parallel)

            if config.perform_initialization:
                # Always initialize bias to zero.
                with torch.no_grad():
                    self.bias.zero_()
        else:
            self.register_parameter('bias', None)

        self._forward_impl = linear_with_grad_accumulation_and_async_allreduce

    def forward(self, input_):
        """Forward of RowParallelLinear

        Args:
            input_: 3D tensor whose order of dimension is [sequence, batch, hidden]

        Returns:
            - output
            - bias
        """
        # Set up backprop all-reduce.
        if self.input_is_parallel:
            input_parallel = input_
        else:
            assert not self.sequence_parallel
            input_parallel = scatter_to_tensor_model_parallel_region(input_)
        # Matrix multiply.
        if not self.weight.requires_grad:
            self._forward_impl = linear_with_frozen_weight
        else:
            self._forward_impl = linear_with_grad_accumulation_and_async_allreduce
        output_parallel = self._forward_impl(
            input=input_parallel,
            weight=self.weight,
            bias=None,
            gradient_accumulation_fusion=self.gradient_accumulation_fusion,
            async_grad_allreduce=False,
            sequence_parallel=False,
        )

        # All-reduce across all the partitions.
        if self.sequence_parallel:
            output_ = reduce_scatter_to_sequence_parallel_region(output_parallel)
        else:
            output_ = reduce_from_tensor_model_parallel_region(output_parallel)
        if not self.skip_bias_add:
            output = output_ + self.bias if self.bias is not None else output_
            output_bias = None
        else:
            output = output_
            output_bias = self.bias
        return output, output_bias
