Add missing support for Flax XLM-RoBERTa
# What does this PR do?

The PyTorch and TF XLM-RoBERTa model and the Flax model for RoBERTa were added a while ago, but there were still a few missing pieces in order to be able to run pre-training and other tasks using Flax XLM-RoBERTa. This PR adds those parts.

Fixes #15910.

## 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.
- [x] 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?

@LysandreJik, @patrickvonplaten 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15900). All of your documentation changes will be reflected on that endpoint.<|||||>Thanks a lot @versae !<|||||>I run the test using the PyTorch weights and it seems to work. I'm also already training a model using a TPUv3-8 with the code in this PR and it seems to be doing fine :) <|||||>Thanks @versae ! Uploaded flax checkpoint for base model, and can confirm that the tests are passing. Merging!