[WIP] Enable GPT2Model to handle 3d attention_mask
This PR solves the problem discussed at #12261.

@patrickvonplaten, @LysandreJik
Hello, thank you for offering a fix! It seems this proposal would be breaking for the GPT-2 double heads model; the following test fails: `test_gpt2_double_lm_head_model` with the following error:

```
_________________ GPT2ModelTest.test_gpt2_double_lm_head_model _________________
[gw1] linux -- Python 3.7.10 /usr/local/bin/python

self = <tests.test_modeling_gpt2.GPT2ModelTest testMethod=test_gpt2_double_lm_head_model>

    def test_gpt2_double_lm_head_model(self):
        config_and_inputs = self.model_tester.prepare_config_and_inputs()
>       self.model_tester.create_and_check_double_lm_head_model(*config_and_inputs)

tests/test_modeling_gpt2.py:457: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
tests/test_modeling_gpt2.py:350: in create_and_check_double_lm_head_model
    result = model(**inputs)
../.local/lib/python3.7/site-packages/torch/nn/modules/module.py:1051: in _call_impl
    return forward_call(*input, **kwargs)
src/transformers/models/gpt2/modeling_gpt2.py:1160: in forward
    return_dict=return_dict,
../.local/lib/python3.7/site-packages/torch/nn/modules/module.py:1051: in _call_impl
    return forward_call(*input, **kwargs)
src/transformers/models/gpt2/modeling_gpt2.py:802: in forward
    output_attentions=output_attentions,
../.local/lib/python3.7/site-packages/torch/nn/modules/module.py:1051: in _call_impl
    return forward_call(*input, **kwargs)
src/transformers/models/gpt2/modeling_gpt2.py:323: in forward
    output_attentions=output_attentions,
../.local/lib/python3.7/site-packages/torch/nn/modules/module.py:1051: in _call_impl
    return forward_call(*input, **kwargs)
src/transformers/models/gpt2/modeling_gpt2.py:258: in forward
    attn_output, attn_weights = self._attn(query, key, value, attention_mask, head_mask)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = GPT2Attention(
  (c_attn): Conv1D()
  (c_proj): Conv1D()
  (attn_dropout): Dropout(p=0.1, inplace=False)
  (resid_dropout): Dropout(p=0.1, inplace=False)
)
query = tensor([[[[ 0.0202, -0.0295, -0.0911,  ..., -0.1715,  0.0008,  0.0310],
          [-0.0724, -0.0315, -0.0237,  ...,  0... -0.0802],
          [-0.1085,  0.0763, -0.0241,  ..., -0.1154,  0.1063, -0.1542]]]],
       grad_fn=<PermuteBackward>)
key = tensor([[[[ 2.2768e-02, -1.5131e-02, -3.1551e-02,  ...,  1.2214e-01,
            3.4581e-03,  1.2902e-01],
          [...6e-01,  1.0495e-01, -8.1176e-02,  ...,  1.5278e-01,
           -1.6426e-01,  4.7595e-02]]]], grad_fn=<PermuteBackward>)
value = tensor([[[[ 0.0396, -0.0556, -0.0115,  ...,  0.1020,  0.0598,  0.1249],
          [ 0.1337, -0.0851,  0.0792,  ...,  0...  0.0443],
          [-0.1049, -0.0717,  0.1128,  ...,  0.2006,  0.0411, -0.0256]]]],
       grad_fn=<PermuteBackward>)
attention_mask = tensor([[[[-10000.,     -0., -10000., -10000.,     -0.,     -0.,     -0.],
          [-10000.,     -0., -10000., -1000...00.,     -0.,     -0., -10000.,     -0.],
          [    -0.,     -0., -10000.,     -0.,     -0., -10000.,     -0.]]]])
head_mask = None

    def _attn(self, query, key, value, attention_mask=None, head_mask=None):
        attn_weights = torch.matmul(query, key.transpose(-1, -2))
    
        if self.scale_attn_weights:
            attn_weights = attn_weights / (float(value.size(-1)) ** 0.5)
    
        if not self.is_cross_attention:
            # if only "normal" attention layer implements causal mask
            query_length, key_length = query.size(-2), key.size(-2)
            causal_mask = self.bias[:, :, key_length - query_length : key_length, :key_length].bool()
            attn_weights = torch.where(causal_mask, attn_weights, self.masked_bias.to(attn_weights.dtype))
    
        if attention_mask is not None:
            # Apply the attention mask
>           attn_weights = attn_weights + attention_mask
E           RuntimeError: The size of tensor a (7) must match the size of tensor b (4) at non-singleton dimension 2

src/transformers/models/gpt2/modeling_gpt2.py:191: RuntimeError
```

Could you give it a look?<|||||>@LysandreJik Thank you for the comment! I inspected code carefully and found that it was because `input_ids` from `create_and_check_double_lm_head_model` has extra dimension for `num_choices` and is merged to `batch` dimension before pushed to model. I fixed this error by adding the condition that `batch_size` of `input_ids` is equal to `batch_size` of `attention_mask` or attention_mask's dimension of `num_choices` would be merged to `batch` dimension as `input_ids`. Also, I added the same code for openai gpt. Please check again!<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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