Add layerdrop to Flaubert
This PR adds `layerdrop` to Flaubert.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2686?src=pr&el=h1) Report
> Merging [#2686](https://codecov.io/gh/huggingface/transformers/pull/2686?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/df27648bd942d59481a13842904f8cb500136e31?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `16.66%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2686/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2686?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2686      +/-   ##
==========================================
- Coverage    74.1%   74.09%   -0.02%     
==========================================
  Files          92       92              
  Lines       15168    15172       +4     
==========================================
+ Hits        11240    11241       +1     
- Misses       3928     3931       +3
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2686?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/2686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2ZsYXViZXJ0LnB5) | `75% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/2686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19mbGF1YmVydC5weQ==) | `29.19% <16.66%> (-0.13%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2686?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2686?src=pr&el=footer). Last update [df27648...15f8b5d](https://codecov.io/gh/huggingface/transformers/pull/2686?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Great, thanks!<|||||>Thanks so much for the quick merge, @LysandreJik!