replace apex.normalization.FusedLayerNorm with torch.nn.LayerNorm
This PR proposes to drop `apex.normalization.FusedLayerNorm` in favor of faster `torch.nn.LayerNorm`. 

1. For performance and background details please see the discussions in https://github.com/huggingface/transformers/issues/9377
2. It's also needed for https://github.com/huggingface/transformers/pull/9384 since `apex.normalization.FusedLayerNorm` corrupts data under model parallel https://github.com/NVIDIA/apex/issues/1022

Fixes: #9377

@LysandreJik, @sgugger, @patrickvonplaten 

Merging since it's blocking #9343 .