Generalize decay_mask_fn to apply mask to all LayerNorm params
# What does this PR do?

Fixes the problem of `decay_mask_fn` not applying mask to all LayerNorm params. 

For example, when running `run_mlm_flax.py` with `roberta-base`'s config, current code fails to apply mask to the LayerNorm's `scale` param of the `lm_head`, 
![Imgur](https://i.imgur.com/87Oy6QJ.png)
This is because `("layer_norm", "scale")` is omitted from `decay_mask_fn`:
```python
def decay_mask_fn(params):
    flat_params = traverse_util.flatten_dict(params)
    flat_mask = {path: (path[-1] != "bias" and path[-2:] != ("LayerNorm", "scale")) for path in flat_params}
    print('flat_mask: ', flat_mask)
    return traverse_util.unflatten_dict(flat_mask)
```

Another example, `run_t5_mlm_flax.py` with `t5-base`'s config omitted all the LayerNorm params, 
![Imgur](https://i.imgur.com/isNQDSf.png) 
This is because `decay_mask_fn` only takes into account `scale` while the T5LayerNorm's param is `weight`.
```python
def decay_mask_fn(params):
    flat_params = traverse_util.flatten_dict(params)
    flat_mask = {
        path: (path[-1] != "bias" and path[-2:] not in [("layer_norm", "scale"), ("final_layer_norm", "scale")])
        for path in flat_params
    }
    print('flat_mask: ', flat_mask)
    return traverse_util.unflatten_dict(flat_mask)
```

## Fix
Generalize `decay_mask_fn` to apply mask to all params whose lowered name containing `layernorm`, `layer_norm` or `ln`.
## Who can review?

potential reviewers: @patrickvonplaten, @sgugger, @patil-suraj

_The documentation is not available anymore as the PR was closed or merged._<|||||>@sanchit-gandhi Thank you for pointing them out! It's been done.<|||||>Amazing, thanks for the PR @duongna21!