add more detailed logging for fp16 diverging
add more detailed logging for fp16 diverging

Summary: We often get a generic "minimum loss scale reached" when fp16 training diverges. Would be useful to have a breakdown on where exactly the gradient norm becomes too big.

Reviewed By: myleott

Differential Revision: D23297774

fbshipit-source-id: 69da1cca1be22f15af633f8efe4e7b491cf4f6f9

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq