Fix LR decay in TF Trainer
Revival of #5051

I have accidentally deleted the previous branch so I recreated this same one.
@LysandreJik Sorry for the inconvenience :(<|||||>Will merge once the code quality passes<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5269?src=pr&el=h1) Report
> Merging [#5269](https://codecov.io/gh/huggingface/transformers/pull/5269?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/0e1fce3c0129d05b65a83cdd89e8eadded553f2e&el=desc) will **decrease** coverage by `0.07%`.
> The diff coverage is `6.25%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5269/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5269?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5269      +/-   ##
==========================================
- Coverage   79.11%   79.03%   -0.08%     
==========================================
  Files         138      138              
  Lines       24080    24102      +22     
==========================================
- Hits        19050    19049       -1     
- Misses       5030     5053      +23     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5269?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/5269/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyX3RmLnB5) | `17.85% <6.25%> (-0.84%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5269/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `79.51% <0.00%> (-1.39%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5269/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.92% <0.00%> (+0.14%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5269?src=pr&el=footer). Last update [0e1fce3...78562e2](https://codecov.io/gh/huggingface/transformers/pull/5269?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>All tests are ok!