Fix a bug in the initialization and serialization of TFRobertaClassificationHead
Author: harkous
Pull request: yes

For `TFRobertaClassificationHead`, `config` was being passed as the first parameter to the `__init__` of the parent class `tf.keras.layers.Layer`. The latter [expects](https://www.tensorflow.org/api_docs/python/tf/keras/layers/Layer) trainable as the first parameter. 

This fixes #4709 and #3664, making the TFRoberta models serializable to `savedmodel` format too.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4884?src=pr&el=h1) Report
> Merging [#4884](https://codecov.io/gh/huggingface/transformers/pull/4884?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/02e5f79662d72cccdca81a47e3001a5f6d36e5b1&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4884/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4884?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4884      +/-   ##
==========================================
+ Coverage   76.46%   76.49%   +0.02%     
==========================================
  Files         128      128              
  Lines       21502    21502              
==========================================
+ Hits        16442    16448       +6     
+ Misses       5060     5054       -6     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4884?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/4884/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `74.74% <100.00%> (ø)` | |
| [src/transformers/modeling\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/4884/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tYXJpYW4ucHk=) | `55.55% <0.00%> (-33.34%)` | :arrow_down: |
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/4884/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `86.48% <0.00%> (-6.31%)` | :arrow_down: |
| [src/transformers/activations.py](https://codecov.io/gh/huggingface/transformers/pull/4884/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9hY3RpdmF0aW9ucy5weQ==) | `85.00% <0.00%> (-5.00%)` | :arrow_down: |
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/4884/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `95.87% <0.00%> (-0.57%)` | :arrow_down: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4884/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `89.46% <0.00%> (-0.24%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4884/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.28% <0.00%> (ø)` | |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4884/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `74.27% <0.00%> (+0.41%)` | :arrow_up: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/4884/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `94.73% <0.00%> (+10.04%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4884?src=pr&el=footer). Last update [02e5f79...9693577](https://codecov.io/gh/huggingface/transformers/pull/4884?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Ahah @LysandreJik you have been too fast :smile: 

Not only this class should have been updated but all that inherit directly from `tf.keras.layers.Layer`. I will do the rest :) 

Thanks a lot @harkous very nice catch!!!

Thanks!
@jplu I tried to verify whether other classes that inherit directly from `tf.keras.layers.Layer` have the same issue but couldn't find any that directly passes `config`. Feel free to double check though.

Great, thanks a lot @jplu !

Oh I didn't know you have already checked that. It is more more than perfect then!! Sorry for my previous post, my bad.