Add support for XLM-R XL and XXL models
Hi,

this PR adds support for the recently released XL and XXL models for XLM-R. These models are described in the ["Larger-Scale Transformers for Multilingual Masked Language Modeling"](https://arxiv.org/abs/2105.00572) paper.

It turns out, that these new models are trained with a more recent version of `fairseq` compared to the "old" XLM-R Base and Large models. Only the current `master` version of `fairseq` is able to load these new models correctly. Unfortunately, some model changes were made (see [this](https://github.com/pytorch/fairseq/commit/54423d3b22a3e7f536e02e9e5445cef9becbd60d) refactoring commit), and the following changes needs also to be done in Transformers library:

The XLM-R Base and Large model used layer normalization in the embeddings, whereas the newer XL and XXL models do not make use of normalized embeddings: layer normalization is done at the end of the transformer. See discussion here: https://github.com/pytorch/fairseq/issues/3600

@patrickvonplaten proposed to introduce a new `RobertaConfig` variable - like `normalize_embeddings` - in order to reflect these model changes in `modeling_roberta.py` directly, instead of writing a new model class (which copies 99% of existing code).

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Changes made so far:

* [x] Update conversion script to work with lastest `fairseq` master version (*1.0.0a*)

Necessary changes:

* [ ] Introduce new config variable in `RobertaConfig` to indicate different layer normalization "strategies"
* [ ] Implement these different layer normalization settings in all modeling classes
* [ ] Re-run conversion script and upload converted XLM-R XL and XXL models to hub
@Soonhwan-Kwon conversion script is currently not working yet for the newer models, but it is working for XLM-R Base for example. Layer norm changes need to be done first in RoBERTa modeling code, so that conversion script is writing a correct model :)<|||||>Hi  @stefan-it thanks for contributing the new models and do you have any plan to push the code and models into https://huggingface.co/models recently @patrickvonplaten ? <|||||>Waiting for this model. Is there any expected timeline? @patrickvonplaten <|||||>Should we try to look into it again @stefan-it ? :-)