

TITLE
Updating outdated fairseq checkpoint to HF script

COMMENTS
1

REACTIONS
+1: 0
-1: 0
laugh: 0
hooray: 0
heart: 0
rocket: 0
eyes: 0

BODY
The current fairseq checkpoint to HF script is outdated, not being compatible with the newly introduced `hydra` config and fairseq's new PyTorch hub interface.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.

