code: def forward(self, x, rel_pos_bias=None):
      B, N, C = x.shape
      qkv_bias = None
      if self.q_bias is not None:
          qkv_bias = torch.cat(
              (
                  self.q_bias,
                  torch.zeros_like(self.v_bias, requires_grad=False),
                  self.v_bias,
              )
          )
      # qkv = self.qkv(x).reshape(
      #    B, N, 3, self.num_heads, C // self.num_heads
      # ).permute(2, 0, 3, 1, 4)
      qkv = F.linear(input=x, weight=self.qkv.weight, bias=qkv_bias)
      qkv = qkv.reshape(B, N, 3, self.num_heads, -1).permute(
          2, 0, 3, 1, 4
      )
      q, k, v = (
          qkv[0],
          qkv[1],
          qkv[2],
      )  # make torchscript happy (cannot use tensor as tuple)
  
      q = q * self.scale
      attn = q @ k.transpose(-2, -1)
  
      if self.relative_position_bias_table is not None:
          relative_position_bias = self.relative_position_bias_table[
              self.relative_position_index.view(-1)
          ].view(
              self.window_size[0] * self.window_size[1] + 1,
              self.window_size[0] * self.window_size[1] + 1,
              -1,
          )  # Wh*Ww,Wh*Ww,nH
          relative_position_bias = relative_position_bias.permute(
              2, 0, 1
          ).contiguous()  # nH, Wh*Ww, Wh*Ww
          attn = attn + relative_position_bias.unsqueeze(0)
  
      if rel_pos_bias is not None:
          attn = attn + rel_pos_bias
  
      attn = attn.softmax(dim=-1)
      attn = self.attn_drop(attn)
  
      x = (attn @ v).transpose(1, 2).reshape(B, N, -1)
      x = self.proj(x)
      x = self.proj_drop(x)
      return x
  
imports: import copy
  import logging
  import math
  from functools import partial
  from typing import List
  import torch
  import torch.nn
  import torch.nn.functional
  from vissl.config import AttrDict
  from vissl.models.model_helpers import DropPath, to_2tuple, trunc_normal_
  from vissl.models.trunks import register_model_trunk
label_desc: Use image node_id 2 for calling the Attention obj's underlying member method code with example usage: obj.forward(x, rel_pos_bias) and returns: x