Update to match renamed attributes in fairseq master
Fix #5917 : RobertaModel no longer have model.encoder and args.num_classes attributes as of 5/28/20.
Do you mind doing the code quality so that we can merge? You can do `pip install -e ".[quality]` followed by `make style && make quality`.

If you don't have access to your fork, I can push on it directly to update the code quality and merge after.<|||||>Hi @LysandreJik,
I tried to do the code quality but couldn't pass `make quality` because `black` and `flake8` return with a non-zero exit code for some reason (i'm on windows). Here are their outputs:
```shell
black --check --line-length 119 --target-version py35 examples templates tests src utils
would reformat C:\Users\u165983\Documents\transformers\src\transformers\__init__.py
would reformat C:\Users\u165983\Documents\transformers\templates\adding_a_new_example_script\run_xxx.py
would reformat C:\Users\u165983\Documents\transformers\templates\adding_a_new_example_script\utils_xxx.py
Oh no! 💥 💔 💥
3 files would be reformatted, 340 files would be left unchanged.

flake8 examples templates tests src utils
tests\test_tokenization_common.py:31:5: F401 'transformers.PretrainedConfig' imported but unused
tests\test_tokenization_common.py:31:5: F401 'transformers.PreTrainedModel' imported but unused
tests\test_tokenization_common.py:31:5: F401 'transformers.TFPreTrainedModel' imported but unused
src\transformers\pipelines.py:72:5: F401 '.modeling_utils.PreTrainedModel' imported but unused
src\transformers\pipelines.py:73:5: F401 '.modeling_tf_utils.TFPreTrainedModel' imported but unused
```
I still commited and pushed on my fork. Tell me if there is anything more I can do.<|||||>`make style` checks `black` and `isort` and updates the files, while `make quality` checks `black`, `isort` and `flake8`, but doesn't update the files and instead tells you what fails.

I ran `make style` on your repository and pushed directly on it, thanks for iterating!