Add TF2 version of FlauBERT
Hello,

I worked today to add the new FlauBERT model in TF2 version. Translated models are available in:
```
jplu/tf-flaubert-base-cased
jplu/tf-flaubert-large-cased
jplu/tf-flaubert-small-cased
jplu/tf-flaubert-base-uncased
```
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2700?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@bae644c`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `75%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2700/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2700?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master    #2700   +/-   ##
=========================================
  Coverage          ?   73.78%           
=========================================
  Files             ?       93           
  Lines             ?    15351           
  Branches          ?        0           
=========================================
  Hits              ?    11326           
  Misses            ?     4025           
  Partials          ?        0
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2700?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/2700/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `21.32% <75%> (ø)` | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2700?src=pr&el=footer). Last update [bae644c...a39b8de](https://codecov.io/gh/huggingface/transformers/pull/2700?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Did we mean to delete camembert imports @LysandreJik . That's why the tests on HEAD are breaking afaict<|||||>I think they were a duplicate