LongformerForMultipleChoice
This PR adds `LongformerForMultipleChoice` following `RobertaForMultipleChoice`

@patrickvonplaten , @ibeltagy 
Same question as before, do we need any automatic global attention here ? 
Haha, I shouldn't have touched this - I kinda already assumed you are working on it :D See PR here: #4645<|||||>Yes 😀 , I was just waiting for `LongformerForTokenClassifiction` to be merged<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4644?src=pr&el=h1) Report
> Merging [#4644](https://codecov.io/gh/huggingface/transformers/pull/4644?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e444648a302dc8520beec96356a4bf500944355c&el=desc) will **decrease** coverage by `2.80%`.
> The diff coverage is `27.58%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4644/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4644?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4644      +/-   ##
==========================================
- Coverage   77.42%   74.62%   -2.81%     
==========================================
  Files         128      128              
  Lines       21017    21046      +29     
==========================================
- Hits        16273    15705     -568     
- Misses       4744     5341     +597     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4644?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/4644/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.13% <ø> (ø)` | |
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/4644/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `73.80% <ø> (ø)` | |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4644/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `19.16% <27.58%> (-77.67%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4644/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `8.72% <0.00%> (-81.21%)` | :arrow_down: |
| [src/transformers/optimization.py](https://codecov.io/gh/huggingface/transformers/pull/4644/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9vcHRpbWl6YXRpb24ucHk=) | `28.00% <0.00%> (-68.00%)` | :arrow_down: |
| [src/transformers/configuration\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4644/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2xvbmdmb3JtZXIucHk=) | `76.92% <0.00%> (-23.08%)` | :arrow_down: |
| [src/transformers/modeling\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/4644/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190NS5weQ==) | `72.81% <0.00%> (-10.72%)` | :arrow_down: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/4644/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `85.71% <0.00%> (-10.00%)` | :arrow_down: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/4644/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `95.94% <0.00%> (-2.71%)` | :arrow_down: |
| ... and [5 more](https://codecov.io/gh/huggingface/transformers/pull/4644/diff?src=pr&el=tree-more) | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4644?src=pr&el=footer). Last update [e444648...5237ac2](https://codecov.io/gh/huggingface/transformers/pull/4644?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Closing in favor of #4645 - sorry should have communicated here better!