Getting no attribute 'output_attentions' error when upgrading to latest huggingface transformers 
# 📚 Migration

## Information

I am getting `torch.nn.modules.module.ModuleAttributeError: 'CaptionBertSelfAttention' object has no attribute 'output_attentions'` error when upgrading my code from pytorch-transformers to latest version of huggingface transformers.

Model I am using (Bert, XLNet ...): Bert

Language I am using the model on (English, Chinese ...): English

The problem arises when using:
* [ ] the official example scripts: (give details below): not sure
* [ ] my own modified scripts: (give details below): yes

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name): no
* [ ] my own task or dataset: (give details below): no

## Details

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I am trying to upgrade my code which uses pytorch-transformers to use latest version of Huggingface transformers. However when I try to use the latest version of huggingface transformers, I get below error:
```
Traceback (most recent call last):
  File "oscar/run_captioning.py", line 1014, in <module>
    main()
  File "oscar/run_captioning.py", line 989, in main
    last_checkpoint = train(args, train_dataloader, val_dataloader, model, tokenizer)
  File "oscar/run_captioning.py", line 479, in train
    outputs = model(**inputs)
  File "/usr/local/lib64/python3.6/site-packages/torch/nn/modules/module.py", line 727, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/home/default/ephemeral_drive/work/image_captioning/Oscar_latest/oscar/modeling/modeling_bert.py", line 450, in forward
    return self.encode_forward(*args, **kwargs)
  File "/home/default/ephemeral_drive/work/image_captioning/Oscar_latest/oscar/modeling/modeling_bert.py", line 458, in encode_forward
    encoder_history_states=encoder_history_states)
  File "/usr/local/lib64/python3.6/site-packages/torch/nn/modules/module.py", line 727, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/home/default/ephemeral_drive/work/image_captioning/Oscar_latest/oscar/modeling/modeling_bert.py", line 281, in forward
    encoder_history_states=encoder_history_states)
  File "/usr/local/lib64/python3.6/site-packages/torch/nn/modules/module.py", line 727, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/home/default/ephemeral_drive/work/image_captioning/Oscar_latest/oscar/modeling/modeling_bert.py", line 115, in forward
    history_state)
  File "/usr/local/lib64/python3.6/site-packages/torch/nn/modules/module.py", line 727, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/home/default/ephemeral_drive/work/image_captioning/Oscar_latest/oscar/modeling/modeling_bert.py", line 146, in forward
    head_mask, history_state)
  File "/usr/local/lib64/python3.6/site-packages/torch/nn/modules/module.py", line 727, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/home/default/ephemeral_drive/work/image_captioning/Oscar_latest/oscar/modeling/modeling_bert.py", line 88, in forward
    self_outputs = self.self(input_tensor, attention_mask, head_mask, history_state)
  File "/usr/local/lib64/python3.6/site-packages/torch/nn/modules/module.py", line 727, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/home/default/ephemeral_drive/work/image_captioning/Oscar_latest/oscar/modeling/modeling_bert.py", line 73, in forward
    outputs = (context_layer, attention_probs) if self.output_attentions else (context_layer,)
  File "/usr/local/lib64/python3.6/site-packages/torch/nn/modules/module.py", line 779, in __getattr__
    type(self).__name__, name))
torch.nn.modules.module.ModuleAttributeError: 'CaptionBertSelfAttention' object has no attribute 'output_attentions'
```

This branch of my code is using the latest version of transformers which gives above error - https://github.com/gsrivas4/Oscar_latest/tree/latest_transformer. This another branch of my code is using older version of transformers (https://github.com/huggingface/transformers/tree/067923d3267325f525f4e46f357360c191ba562e) which runs without any error - https://github.com/gsrivas4/Oscar_latest/tree/old_transformers. I have added README.md files to run both the branches.

So far, based on my debugging the issue I understand that `self.output_attentions` is defined in the older version of transformers here - https://github.com/huggingface/transformers/blob/067923d3267325f525f4e46f357360c191ba562e/pytorch_transformers/modeling_bert.py#L281. However, in the latest version of transformers `self.output_attentions` is not defined in the class `BertSelfAttention` - https://github.com/huggingface/transformers/blob/master/src/transformers/models/bert/modeling_bert.py#L213-L236. As `self.output_attentions` is not defined the latest version of transformers, which causes the error.

I have checked the migration document and did not the find steps needed or guidelines about how to resolve the issue  caused by upgrading huggingface transformers - https://huggingface.co/transformers/migration.html#migrating-from-transformers-v3-x-to-v4-x.

It would be really helpful to know how to resolve the error.

## Environment info
<!-- You can run the command `python transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
 
- `transformers` version: https://github.com/huggingface/transformers
- Platform: x86_64 GNU/Linux
- Python version: 3.6.8
- PyTorch version (GPU?): 1.7.0+cu101 (GPU)
- Tensorflow version (GPU?): 2.3.0 (GPU)
- Using GPU in script?: yes
- Using distributed or parallel set-up in script?: no

<!-- IMPORTANT: which version of the former library do you use? -->
* `pytorch-transformers` or `pytorch-pretrained-bert` version (or branch): https://github.com/huggingface/transformers/tree/067923d3267325f525f4e46f357360c191ba562e

## Checklist

- [ yes] I have read the migration guide in the readme.
 ([pytorch-transformers](https://github.com/huggingface/transformers#migrating-from-pytorch-transformers-to-transformers);
  [pytorch-pretrained-bert](https://github.com/huggingface/transformers#migrating-from-pytorch-pretrained-bert-to-transformers))
- [yes ] I checked if a related official extension example runs on my machine.

#### Update: 
I changed the following line of code from 
`outputs = (context_layer, attention_probs) if self.output_attentions else (context_layer,)` to 
 `outputs = (context_layer,)` 
and my code seems to run fine with the latest transformers - https://github.com/gsrivas4/Oscar_latest/blob/latest_transformer/oscar/modeling/modeling_bert.py#L74-L75. However, I am still not sure if this change can break something in code logically.<|||||>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.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.