[apex.normalizations.FusedLayerNorm] torch.cuda.is_available() is redundant as apex handles that internally
This PR is a follow up to https://github.com/huggingface/transformers/issues/9338

According to https://github.com/huggingface/transformers/issues/9338#issuecomment-752242098 we can just remove the `torch.cuda.is_available()` check before importing `apex.normalizations.FusedLayerNorm` and the multiprocess problem will go away.

Fixes #9338

@patrickvonplaten 
Thanks a lot for digging into this @stas00 