code: def _merge_heads(self, tensor, num_heads, attn_head_size):
      tensor = tensor.permute(0, 2, 1, 3).contiguous()
      new_shape = tensor.size()[:-2] + (num_heads * attn_head_size,)
      return tensor.view(new_shape)
  
imports: from typing import Optional, Tuple, Union
  import torch
  import torch.nn
  from transformers.activations import ACT2FN
  from transformers.models.gpt2.modeling_gpt2 import BaseModelOutputWithPastAndCrossAttentions, GPT2PreTrainedModel
  from transformers.pytorch_utils import Conv1D
label_desc: Use image node_id 4 for calling the GPT2Attention obj's underlying member method code with example usage: obj._merge_heads(tensor, num_heads, attn_head_size) and returns: tensor