Fix boundary condition in `--patience` (#1845)
Fix boundary condition in `--patience` (#1845)

Summary:
When `--patience=1`, we should stop as soon as the validation loss decreases.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/1845

Differential Revision: D20468392

Pulled By: myleott

fbshipit-source-id: 5ee7741b5086536c48d7b804c7b571749b464f3f

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq