Flax XLM-RoBERTa
# 🌟 New model addition: Flax XLM-RoBERTa

## Model description

The Flax version of RoBERTa and PyTorch and TF XLM-RoBERTa have been there for a while, but some pieces were missing that prevented us from doing pre-training and other tasks on Flask XLM-RoBERTa. In this PR #15900, I am adding those parts that were necessary to support XLM-RoBERTa in Flax based on the existing XLM-RoBERTa and Flax RoBERTa implementations.

## Open source status

* [x] the model implementation is available: it is based off the existing implementation of XLM-RoBERTa  and Flax RoBERTa in HuggingFace.
* [x] the model weights are available: not as Flax, but loading from PyTorch seems to work.
* [x] who are the authors: @aconneau I think, but not sure who ported it to HF. Maybe @sgugger, @patrickvonplaten?
