ElectraForMultipleChoice
This PR add `ElectraForMultipleChoice`. One of the missing models in this [project](https://github.com/huggingface/transformers/projects/17).

Since, for multiple choice pooled outputs are needed, added `ElectraPooler` class.

@sgugger , @LysandreJik 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4954?src=pr&el=h1) Report
> Merging [#4954](https://codecov.io/gh/huggingface/transformers/pull/4954?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/efeb75b8054cc299698cf8bc09f395ada2660745&el=desc) will **increase** coverage by `0.08%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4954/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4954?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4954      +/-   ##
==========================================
+ Coverage   77.24%   77.33%   +0.08%     
==========================================
  Files         133      133              
  Lines       22134    22166      +32     
==========================================
+ Hits        17097    17141      +44     
+ Misses       5037     5025      -12     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4954?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/4954/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.16% <ø> (ø)` | |
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/4954/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `70.76% <ø> (ø)` | |
| [src/transformers/configuration\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/4954/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2VsZWN0cmEucHk=) | `100.00% <100.00%> (ø)` | |
| [src/transformers/modeling\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/4954/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbGVjdHJhLnB5) | `80.12% <100.00%> (+1.95%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4954/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.06% <0.00%> (+0.12%)` | :arrow_up: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/4954/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.62% <0.00%> (+0.23%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4954/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.57% <0.00%> (+1.55%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4954?src=pr&el=footer). Last update [efeb75b...9cb71f9](https://codecov.io/gh/huggingface/transformers/pull/4954?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>hi @LysandreJik  could you tell me why these tests are failing now ? Thanks.<|||||>Looks like @LysandreJik deleted a function by mistake during the merge (the `create_and_check_electra_for_multiple_choice`), could you add it back?<|||||>My bad, sorry about that. Thanks for the fix!