Refactoring the TF activations functions
Author: jplu
Pull request: yes

This PR groups all the TF activations functions into a common file like it is done in the PT part.


Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7150?src=pr&el=h1) Report
> Merging [#7150](https://codecov.io/gh/huggingface/transformers/pull/7150?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/52d250f6aa14844024806e5e4dd1c7882bbd8dd5?el=desc) will **increase** coverage by `0.38%`.
> The diff coverage is `88.52%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7150/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7150?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7150      +/-   ##
==========================================
+ Coverage   79.12%   79.50%   +0.38%     
==========================================
  Files         168      169       +1     
  Lines       32303    32287      -16     
==========================================
+ Hits        25560    25671     +111     
+ Misses       6743     6616     -127     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7150?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/7150/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG5ldC5weQ==) | `20.85% <50.00%> (-71.33%)` | :arrow_down: |
| [src/transformers/activations\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/7150/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9hY3RpdmF0aW9uc190Zi5weQ==) | `75.00% <75.00%> (ø)` | |
| [src/transformers/modeling\_tf\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/7150/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hbGJlcnQucHk=) | `90.92% <100.00%> (+0.01%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/7150/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `98.90% <100.00%> (+0.16%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/7150/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `99.28% <100.00%> (+65.24%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/7150/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `98.95% <100.00%> (+<0.01%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_funnel.py](https://codecov.io/gh/huggingface/transformers/pull/7150/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mdW5uZWwucHk=) | `94.04% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/7150/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `71.59% <100.00%> (-23.42%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_lxmert.py](https://codecov.io/gh/huggingface/transformers/pull/7150/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9seG1lcnQucHk=) | `94.55% <100.00%> (+0.43%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/7150/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `97.06% <100.00%> (+0.15%)` | :arrow_up: |
| ... and [30 more](https://codecov.io/gh/huggingface/transformers/pull/7150/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7150?src=pr&el=footer). Last update [52d250f...a639d90](https://codecov.io/gh/huggingface/transformers/pull/7150?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).