Correct TF formatting to exclude LayerNorms from weight decay
Fixes #4360 

Layer Norm is formated in the wrong way for TensorFlow. This causes it not to be excluded from weight decay in the [run_tf_glue.py](https://github.com/huggingface/transformers/blob/master/examples/text-classification/run_tf_glue.py) script. 

This pr simply formats the string to fit the TensorFlow naming. 

Not sure if you want a test for this? One option would be to check that all elements in `_exclude_from_weight_decay` trigger a regexp match. But seems a bit overkill. 
Hello!

Thanks for the fix! Just one suggestion above :)<|||||>Thanks for the feedback! Sounds like a good idea, added that. Are the failing ci tests an issue? Everything passes on my machine. <|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4448?src=pr&el=h1) Report
> Merging [#4448](https://codecov.io/gh/huggingface/transformers/pull/4448?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/2d184cb553ee20943b03b253f44300e466357871&el=desc) will **increase** coverage by `0.84%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4448/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4448?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4448      +/-   ##
==========================================
+ Coverage   77.30%   78.14%   +0.84%     
==========================================
  Files         120      120              
  Lines       20027    20027              
==========================================
+ Hits        15481    15651     +170     
+ Misses       4546     4376     -170     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4448?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/optimization\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/4448/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9vcHRpbWl6YXRpb25fdGYucHk=) | `79.24% <ø> (ø)` | |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4448/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.44% <0.00%> (-0.42%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4448/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.08% <0.00%> (+0.12%)` | :arrow_up: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/4448/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `43.25% <0.00%> (+1.10%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/4448/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `81.78% <0.00%> (+1.37%)` | :arrow_up: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/4448/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `75.73% <0.00%> (+2.29%)` | :arrow_up: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/4448/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `97.81% <0.00%> (+2.62%)` | :arrow_up: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/4448/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `95.71% <0.00%> (+10.00%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4448/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `89.93% <0.00%> (+81.20%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4448?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4448?src=pr&el=footer). Last update [2d184cb...d99d65c](https://codecov.io/gh/huggingface/transformers/pull/4448?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Awesome! LGTM :)

/cc @julien-c and @LysandreJik 