Roberta token classification
Roberta is missing the token classification that is already available in the BERT models. Per discussion in #1166 they mention it should be more or less a copy paste of the current `BertForTokenClassification` and `TFBertForTokenClassification `. I noticed this is also missing as I hope to update the `run_ner.py` file to include DistilRoberta, which needs these new classes (#1534).

Changes
* Simple copy paste of the related Bert models for Roberta. I added tests that also reflect the same changes. Minor tweaks were made that are different in the Roberta models (inheriting from Bert and changing the configs to Roberta).

Tests seem to pass but as this is my first PR, I would like some more feedback if this in fact works correctly.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1613?src=pr&el=h1) Report
> Merging [#1613](https://codecov.io/gh/huggingface/transformers/pull/1613?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/5b6cafb11b39e78724dc13b57b81bd73c9a66b49?src=pr&el=desc) will **decrease** coverage by `0.27%`.
> The diff coverage is `23.72%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1613/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1613?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master   #1613      +/-   ##
=========================================
- Coverage   86.17%   85.9%   -0.28%     
=========================================
  Files          91      91              
  Lines       13595   13653      +58     
=========================================
+ Hits        11715   11728      +13     
- Misses       1880    1925      +45
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1613?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/tests/modeling\_tf\_roberta\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1613/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX3JvYmVydGFfdGVzdC5weQ==) | `75.2% <14.28%> (-3.62%)` | :arrow_down: |
| [transformers/tests/modeling\_roberta\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1613/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3JvYmVydGFfdGVzdC5weQ==) | `75.38% <22.22%> (-4.13%)` | :arrow_down: |
| [transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/1613/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3JvYmVydGEucHk=) | `71.25% <25%> (-9.32%)` | :arrow_down: |
| [transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/1613/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3JvYmVydGEucHk=) | `90.67% <26.66%> (-9.33%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/1613?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/1613?src=pr&el=footer). Last update [5b6cafb...d555603](https://codecov.io/gh/huggingface/transformers/pull/1613?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Closing this as superseded by #1625