Update modeling_electra for adding ability to using electra as decoder
Author: blizda
Pull request: yes
Labels: name: wontfix

Adapt electra for using as decoder(code for it borrowed from modeling_bert)

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4651?src=pr&el=h1) Report
> Merging [#4651](https://codecov.io/gh/huggingface/transformers/pull/4651?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e444648a302dc8520beec96356a4bf500944355c&el=desc) will **decrease** coverage by `0.29%`.
> The diff coverage is `20.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4651/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4651?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4651      +/-   ##
==========================================
- Coverage   77.42%   77.13%   -0.30%     
==========================================
  Files         128      128              
  Lines       21017    21041      +24     
==========================================
- Hits        16273    16229      -44     
- Misses       4744     4812      +68     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4651?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/4651/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19lbGVjdHJhLnB5) | `71.11% <20.00%> (-5.32%)` | :arrow_down: |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/4651/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `72.11% <0.00%> (-14.11%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/4651/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.41% <0.00%> (-1.38%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4651/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `89.94% <0.00%> (-0.24%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4651/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.66% <0.00%> (+0.16%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4651/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (+0.41%)` | :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/4651?src=pr&el=footer). Last update [e444648...9660de8](https://codecov.io/gh/huggingface/transformers/pull/4651?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


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