Updating outdated fairseq checkpoint to HF script
The current fairseq checkpoint to HF script is outdated, not being compatible with the newly introduced `hydra` config and fairseq's new PyTorch hub interface.  In addition to this, added one more argument (`--data-dir`) for custom RoBERTa models, and modified the `--classification_head` argument to take in a string rather than `store_true`. This is to reflect (a more likely case) of custom classification heads, rather than the most popular (and already available) `mnli` head. Added an import of `os` (if it counts as a "dependency").

## 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).
- [ ] Did you write any new necessary tests?

## Who can review?
@LysandreJik @stefan-it @myleott @sgugger 

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors which may be interested in your PR.


This issue has been automatically marked as stale and been closed because it has not had recent activity. Thank you for your contributions.

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