

TITLE
Add RemBERT model code to huggingface

COMMENTS
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BODY
Add RemBERT model to Huggingface ( https://arxiv.org/abs/2010.12821 ).

This adds code to support the RemBERT model in Huggingface.

In terms of implementation, this is roughly a scaled up version of mBERT with ALBERT-like factorized embeddings and tokenizer.

Still needs to be done:
- [x] Check results validity
- [x] Upload model to model hub
- [x] FastTokenizer version
- [x]  More testing
- [x] TF code

Fixes #9711 


## Before submitting
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [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 seems appropriate here. 