Fix `RobertaClassificationHead` style consistency.
Author: ranamihir
Pull request: yes

There's a slight inconsistency in `RobertaClassificationHead` in that it takes in the whole sequence output from the `RobertaModel`, and extracts the pooled output inside its own forward method, seen [here](https://github.com/huggingface/transformers/blob/d713cfc5ebfb1ed83de1fce55dd7279f9db30672/src/transformers/modeling_roberta.py#L573).


This is different from other models, where the pooled output is computed beforehand and directly passed on to the classifier. E.g. in [`BertForSequenceClassification`](https://github.com/huggingface/transformers/blob/d713cfc5ebfb1ed83de1fce55dd7279f9db30672/src/transformers/modeling_bert.py#L1147), [`DistilBertForSequenceClassification`](https://github.com/huggingface/transformers/blob/d713cfc5ebfb1ed83de1fce55dd7279f9db30672/src/transformers/modeling_distilbert.py#L614), [`BartForSequenceClassification`](https://github.com/huggingface/transformers/blob/d713cfc5ebfb1ed83de1fce55dd7279f9db30672/src/transformers/modeling_bart.py#L1097), etc.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4107?src=pr&el=h1) Report
> Merging [#4107](https://codecov.io/gh/huggingface/transformers/pull/4107?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/58cca47c16149e43d1b516623d59e3c5d97f695e&el=desc) will **decrease** coverage by `1.21%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4107/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4107?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4107      +/-   ##
==========================================
- Coverage   77.83%   76.62%   -1.22%     
==========================================
  Files         141      141              
  Lines       24634    24634              
==========================================
- Hits        19175    18876     -299     
- Misses       5459     5758     +299     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4107?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/4107/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `95.00% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/4107/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `93.04% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/4107/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `26.92% <0.00%> (-68.47%)` | :arrow_down: |
| [src/transformers/tokenization\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/4107/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtLnB5) | `16.26% <0.00%> (-66.67%)` | :arrow_down: |
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/4107/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `68.14% <0.00%> (-25.67%)` | :arrow_down: |
| [src/transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/4107/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdHJhbnNmb194bC5weQ==) | `33.56% <0.00%> (-8.93%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4107/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `82.95% <0.00%> (-2.26%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/4107/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `92.44% <0.00%> (-1.17%)` | :arrow_down: |
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/4107/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `75.82% <0.00%> (-0.18%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/4107/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `98.76% <0.00%> (+32.51%)` | :arrow_up: |

------

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