Clarify mixed precision training support (#766)
Clarify mixed precision training support (#766)

Summary:
Change the wording to avoid confusion. Mixed precision ensures both higher arithmetic throughput and numerical stability, not exactly synonymous to pure half-precision/FP16 training. Also add mentioning of tensor cores since older generation GPUs without tensor cores don't support true mixed precision training.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/766

Differential Revision: D15559565

Pulled By: myleott

fbshipit-source-id: c71e720772657bb3e8ad330b58bf69e23beb614e

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq