Add SequenceClassification and MultipleChoice TF models to Electra
This PR adds SequenceClassification and MultipleChoice TF models to Electra. It is a follow up to the PR https://github.com/huggingface/transformers/pull/4654.

@LysandreJik is it possible to add `"summary_proj_to_labels": true` to the Electra config?
@LysandreJik I don't understand why the Torch tests are trying to execute a TF test :thinking: 

```
______________ ERROR collecting tests/test_modeling_tf_electra.py ______________
ImportError while importing test module '/home/circleci/transformers/tests/test_modeling_tf_electra.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/local/lib/python3.7/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/test_modeling_tf_electra.py:19: in <module>
    import tensorflow as tf
E   ModuleNotFoundError: No module named 'tensorflow'
```

BTW I finally decided to mirror the behavior of the PT version of these models and not touching to the configuration in order to avoid probable bugs it might bring on that side.<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6227?src=pr&el=h1) Report
> Merging [#6227](https://codecov.io/gh/huggingface/transformers/pull/6227?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/0513f8d275022d4055b710a33cd520b2000982bf&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `79.31%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6227/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6227?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6227      +/-   ##
==========================================
+ Coverage   79.61%   79.63%   +0.02%     
==========================================
  Files         146      146              
  Lines       26597    26683      +86     
==========================================
+ Hits        21175    21250      +75     
- Misses       5422     5433      +11     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6227?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/6227/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.24% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/6227/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hdXRvLnB5) | `66.66% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/6227/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `91.54% <79.31%> (-3.99%)` | :arrow_down: |
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6227/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `68.14% <0.00%> (-25.67%)` | :arrow_down: |
| [src/transformers/tokenization\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/6227/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtX3JvYmVydGEucHk=) | `84.52% <0.00%> (-10.72%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6227/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.29% <0.00%> (+0.32%)` | :arrow_up: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6227/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.46% <0.00%> (+4.76%)` | :arrow_up: |
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6227/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `95.77% <0.00%> (+35.21%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6227?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6227?src=pr&el=footer). Last update [0513f8d...90c9f35](https://codecov.io/gh/huggingface/transformers/pull/6227?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Great, thanks! I'll let @sgugger merge it before/after #6247 to make sure he doesn't get nasty conflicts.<|||||>Merging it before and praying!