Merge internal changes (#483)

Summary:
Changelog:
- `4889802`: can now remove detokenize sentencepiece output with `--remove-bpe=sentencepiece` (fixes #331). Also added `--sacrebleu` for computing detokenized BLEU.
- `0d76427`: fix assertion error when training language model with dataset containing empty sentences
- minor bug and style fixes
Pull Request resolved: https://github.com/pytorch/fairseq/pull/483

Differential Revision: D13867899

Pulled By: myleott

fbshipit-source-id: 25c940b847fe270262ac8f5ac838407b3977fdda