Replace BertLayerNorm with LayerNorm
Running Movement pruning experiments with the newest HuggingFace would crash due to non-existing BertLayerNorm.
@VictorSanh @eldarkurtic 

Hello guys!

Thanks for your effort for such a great repo!

I'm wondering why the implementation of LayerNorm in BERT seems not to be identical to what's commonly discussed version of `LayerNorm`.
I've read several tutorials about `LayerNorm`, and according to their explanation, the implementation seems to be like following:
```
input_x = torch.rand(batch_size, sequence_length, hidden_size)
layer_norm = torch.nn.LayerNorm([sequence_length, hidden_size])
output = layer_norm(input_x)
```
In above implementation, the normalization is calculated through the dim of `sequence_length` as well as `hidden_size`, but the implementation in this repo only normalize the tensors in the last dim, which I think is somehow identical to `InstanceNorm`?