Add AlbertForMultipleChoice
Another [model missing](https://github.com/huggingface/transformers/projects/17).

While implementing it I noticed two things:
- the example in `BertForMultipleChoice` wasn't working, so fixed it.
- some model classes where missing in the `all_model_classes` in the test, fixed it for albert and bert, will look at the other tests in a separate PR dedicated to that.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4885?src=pr&el=h1) Report
> Merging [#4885](https://codecov.io/gh/huggingface/transformers/pull/4885?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/f0340b30310cb78555c6f78bed7262101f251940&el=desc) will **increase** coverage by `0.10%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4885/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4885?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4885      +/-   ##
==========================================
+ Coverage   76.47%   76.57%   +0.10%     
==========================================
  Files         128      128              
  Lines       21502    21528      +26     
==========================================
+ Hits        16443    16486      +43     
+ Misses       5059     5042      -17     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4885?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/4885/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.13% <ø> (ø)` | |
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/4885/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `73.60% <ø> (ø)` | |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/4885/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `86.27% <ø> (ø)` | |
| [src/transformers/modeling\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/4885/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hbGJlcnQucHk=) | `77.68% <100.00%> (+1.34%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/4885/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `79.58% <0.00%> (-1.39%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4885/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.28% <0.00%> (ø)` | |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/4885/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `94.73% <0.00%> (+10.04%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4885?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/4885?src=pr&el=footer). Last update [f0340b3...09b0fd5](https://codecov.io/gh/huggingface/transformers/pull/4885?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>This needs rework now that #4921 has been merged, so do not merge just yet.<|||||>Ugh, rebase went wrong, closing...