Fix to keep up with the changes made in fairseq's RobertaModel
Author: hmirin
Pull request: yes

# What does this PR do?

- This PR fixes broken model conversion script.
  - Current script seems to be outdated with the latest release and main branch fairseq.
    - `RobertaModel.from_pretrained` now returns RobertaHubInterface, which seems to be different from the assumption of the current implementation.
    - cf. https://github.com/pytorch/fairseq/blob/v0.10.2/fairseq/models/roberta/model.py#L256

- This PR also make this script to use the positional encoding size from the model, not the hardcoded value.

Current implementation causes errors like: 
```
Traceback (most recent call last):
  File "/usr/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/usr/local/lib/python3.7/dist-packages/transformers/models/roberta/convert_roberta_original_pytorch_checkpoint_to_pytorch.py", line 177, in <module>
    args.roberta_checkpoint_path, args.pytorch_dump_folder_path, args.classification_head
  File "/usr/local/lib/python3.7/dist-packages/transformers/models/roberta/convert_roberta_original_pytorch_checkpoint_to_pytorch.py", line 59, in convert_roberta_checkpoint_to_pytorch
    hidden_size=roberta.args.encoder_embed_dim,
  File "/usr/local/lib/python3.7/dist-packages/torch/nn/modules/module.py", line 1131, in __getattr__
    type(self).__name__, name))
AttributeError: 'RobertaHubInterface' object has no attribute 'args'
```

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
  - There is no mention of this script in the documentation.
- [ ] Did you write any new necessary tests?
  - There is no test of this script in the repository.


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

Ping to the contributors of this script:
- @LysandreJik
- @sgugger



Comments:
I just ran the script as it is currently with the lastest pypi available fairseq and it converted the model successfully:

```
torch.Size([1, 11, 50265]) torch.Size([1, 11, 50265])
max_absolute_diff = 0.0
Do both models output the same tensors? 🔥
Saving model to here
Configuration saved in here/config.json
Model weights saved in here/pytorch_model.bin
```



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