Fix TF WarmUp class
This commit fixes the TF WarmUp learning rate scheduler.

The LR shape was wrong due to warmup steps. See linked issue for more details.

Fix #5098
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5099?src=pr&el=h1) Report
> Merging [#5099](https://codecov.io/gh/huggingface/transformers/pull/5099?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/efeb75b8054cc299698cf8bc09f395ada2660745&el=desc) will **increase** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5099/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5099?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5099      +/-   ##
==========================================
+ Coverage   77.24%   77.29%   +0.04%     
==========================================
  Files         133      133              
  Lines       22134    22134              
==========================================
+ Hits        17097    17108      +11     
+ Misses       5037     5026      -11     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5099?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/optimization\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/5099/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9vcHRpbWl6YXRpb25fdGYucHk=) | `57.27% <ø> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5099/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.06% <0.00%> (+0.12%)` | :arrow_up: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5099/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.62% <0.00%> (+0.23%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5099/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `75.40% <0.00%> (+0.40%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5099/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.26% <0.00%> (+1.24%)` | :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/5099?src=pr&el=footer). Last update [efeb75b...d99033f](https://codecov.io/gh/huggingface/transformers/pull/5099?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Thanks @Colanim! Good catch!! Unfortunately it is a duplicate of #4940 :smile: and should be merged soon :)<|||||>Haha didn't see x)