Add --fp16-scale-tolerance (#397)

Summary:
Let's only decrease the loss scale if a large enough percentage of batches overflow.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/397

Differential Revision: D13355159

Pulled By: myleott

fbshipit-source-id: e17dde73d34a639519b4348c013fdd19d2b314e6