Adding required flags to non-default arguments in hf_argparser
Signed-off-by: Adam Pocock <adam.pocock@oracle.com>

# What does this PR do?

Fixes #10677. I didn't update the docs as I think this is the intended behaviour, but I can do if you think this change would be unexpected.

## 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?
- [x] 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. Issue #10677.
- [ ] 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).
- [x] Did you write any new necessary tests?


## Who can review?

@sgugger
No problem. I can't see what test failed in CircleCI as it wants to bind to my Github account and access private things in my org. Is there any way to see the failures without letting it into my account?