LayerNorm not excluded from weight decay in TF
# 🐛 Bug

## Information

Model I am using (Bert, XLNet ...):
bert-base-cased

Language I am using the model on (English, Chinese ...):
English

The problem arises when using:
* [X] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [X] an official GLUE/SQUaD task: (give the name)
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. Add a print statement to `_do_use_weight_decay` in [AdamWeightDecay](https://github.com/huggingface/transformers/blob/master/src/transformers/optimization_tf.py) to see which parameters are actually excluded:
```python
    def _do_use_weight_decay(self, param_name):
        """Whether to use L2 weight decay for `param_name`."""
        if self.weight_decay_rate == 0:
            return False

        if self._include_in_weight_decay:
            for r in self._include_in_weight_decay:
                if re.search(r, param_name) is not None:
                    return True

        if self._exclude_from_weight_decay:
            for r in self._exclude_from_weight_decay:
                if re.search(r, param_name) is not None:
                    print(f"Found: {param_name}")
                    return False
        return True
```
2. run `python examples/text-classification/run_tf_glue.py --model_name_or_path bert-base-cased --task_name mrpc --output_dir temp --logging_dir temp --do_train --overwrite_output_dir --optimizer_name adamw`. 
3. Observe that no weights related to layer norms are printed. 

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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->
The weights of the layer norms (and the biases) should be printed. 

See for example: https://github.com/google-research/bert/blob/f39e881b169b9d53bea03d2d341b31707a6c052b/optimization.py. 

Based on the fact that no layer norm weights are printed with "layer_norm" simply switching "layer_norm" to "LayerNorm" seems like the easiest change. 

## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 2.9.0
- Platform: Darwin-19.4.0-x86_64-i386-64bit
- Python version: 3.7.7
- PyTorch version (GPU?): 1.5.0 (False)
- Tensorflow version (GPU?): 2.2.0 (False)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

Thanks for the issue dude! Feel free to open a PR if you have a branch that fixed that lying around haha ;-)  