Add FlauBERT: Unsupervised Language Model Pre-training for French
This PR adds [FlauBERT](https://github.com/getalp/Flaubert). Most of the code is derived from XLM (there are some new features in FlauBERT such as `pre_norm` and `layerdrop`).

`make test` had 1 failure related to BERT and not to FlauBERT:

> [gw0] FAILED tests/test_configuration_auto.py::AutoConfigTest::test_pattern_matching_fallback

`make style` passed.

`make quality` passed.
Hi, I don't really know how it happened but I was denied push access on your repository while patching the failing FlauBERT bug. Instead I pushed to a new branch `flaubert` on this remote (huggingface/transformers), and I'm opening a pull request with your changes.

You're still the author of the commit.<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2632?src=pr&el=h1) Report
> Merging [#2632](https://codecov.io/gh/huggingface/transformers/pull/2632?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/adb8c93134f02fd0eac2b52189364af21977004c?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2632/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2632?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2632   +/-   ##
=======================================
  Coverage   74.59%   74.59%           
=======================================
  Files          89       89           
  Lines       14971    14971           
=======================================
  Hits        11168    11168           
  Misses       3803     3803
```



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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2632?src=pr&el=footer). Last update [adb8c93...adb8c93](https://codecov.io/gh/huggingface/transformers/pull/2632?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>The PR is #2677. I'm updating the documentation directly on this PR.