Fix AutoModelForQuestionAnswering for Roberta
When using `AutoModelForQuestionAnswering()` to load a Roberta model, we are currently instantiating a `BertForQuestionAnswering` class. This is happening because `RobertaConfig` is an instance of `BertConfig` (due to inheritance) and there's no other mapping for Roberta in here:

https://github.com/huggingface/transformers/blob/bac51fba3a6b96f02f482e9a352601242b200e47/src/transformers/modeling_auto.py#L176-L184



# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2655?src=pr&el=h1) Report
> Merging [#2655](https://codecov.io/gh/huggingface/transformers/pull/2655?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/babd41e7fa07bdd764f8fe91c33469046ab7dbd1?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2655/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2655?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2655   +/-   ##
=======================================
  Coverage   74.58%   74.58%           
=======================================
  Files          87       87           
  Lines       14892    14892           
=======================================
  Hits        11107    11107           
  Misses       3785     3785
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2655?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/2655/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `75.55% <ø> (ø)` | :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/2655?src=pr&el=footer). Last update [babd41e...213877a](https://codecov.io/gh/huggingface/transformers/pull/2655?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>can you run `make style` @tholor? Seems like I can't push to your fork or I'd have done it myself.

Thank you!<|||||>Actually I'll do it myself. Thanks!