Add missing imports
Add missing imports

Summary: Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/579

Differential Revision: D15352058

Pulled By: myleott

fbshipit-source-id: cebef02edcfcb203ef2e32c64f7f28e08c4e46b0

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq