Question Answering support for Albert and Roberta in TF
This PR simply adds `TFRobertaForQuestionAnswering` and `TFAlbertForQuestionAnswering` classes (I needed them to do some model conversions!)
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3812?src=pr&el=h1) Report
> Merging [#3812](https://codecov.io/gh/huggingface/transformers/pull/3812?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/301bf8d1b43d99efe1fdb5ba15871e975b3cb6cf&el=desc) will **increase** coverage by `0.04%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3812/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3812?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3812      +/-   ##
==========================================
+ Coverage   78.27%   78.31%   +0.04%     
==========================================
  Files         106      106              
  Lines       17964    17996      +32     
==========================================
+ Hits        14061    14094      +33     
+ Misses       3903     3902       -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3812?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/3812/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `98.98% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/3812/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hdXRvLnB5) | `68.62% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/3812/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hbGJlcnQucHk=) | `86.25% <100.00%> (+0.67%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/3812/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `100.00% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3812/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `93.00% <0.00%> (+0.16%)` | :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/3812?src=pr&el=footer). Last update [301bf8d...44c92f3](https://codecov.io/gh/huggingface/transformers/pull/3812?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
