Add DeeBERT (entropy-based early exiting for *BERT)
Add DeeBERT (entropy-based early exiting for *BERT).
Paper: https://www.aclweb.org/anthology/2020.acl-main.204/
Based on its original repository: https://github.com/castorini/DeeBERT 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5477?src=pr&el=h1) Report
> Merging [#5477](https://codecov.io/gh/huggingface/transformers/pull/5477?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/58cca47c16149e43d1b516623d59e3c5d97f695e&el=desc) will **increase** coverage by `0.41%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5477/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5477?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5477      +/-   ##
==========================================
+ Coverage   77.83%   78.25%   +0.41%     
==========================================
  Files         141      141              
  Lines       24634    24634              
==========================================
+ Hits        19175    19278     +103     
+ Misses       5459     5356     -103     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5477?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5477/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `43.47% <0.00%> (-49.57%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5477/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `81.09% <0.00%> (+1.37%)` | :arrow_up: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5477/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.71% <0.00%> (+1.50%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5477/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `94.98% <0.00%> (+74.19%)` | :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/5477?src=pr&el=footer). Last update [58cca47...f44de41](https://codecov.io/gh/huggingface/transformers/pull/5477?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Btw: would be awesome so see a token classification example 😅<|||||>Hi @JetRunner, thanks for the review! I have updated according to your suggestions.<|||||>2 checks fail, however they don't seem relevant to my commits.<|||||>@LysandreJik Thanks for the comments and I've updated accordingly!