Manually port pull request 385

Summary:
Manually port fairinternal fairseq-py pull request #385 [1] to fbcode.

Resolve the merge conflict of removing fp16_trainer per offline discussion with Myle. Also updated codes to make generate.py works.

[1] https://github.com/fairinternal/fairseq-py/pull/385/commits/18fa6e154781cf0c4b1596429dba7e753a545069

Reviewed By: liezl200

Differential Revision: D10052908

fbshipit-source-id: c3c378d78dc1e9ac087c815f359e78c0048ff2f5