Fix to keep up with the changes made in fairseq's RobertaModel
# 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  - Current script seems to be outdated with the latest release and main branch fairseq.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


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
```

<|||||>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.