Add Dictionary.pad_to_multiple_ helper
Add Dictionary.pad_to_multiple_ helper

Summary: Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/970

Differential Revision: D19351834

Pulled By: myleott

fbshipit-source-id: 7149973728ef7595c16de7ac0c8b71747b89f65b

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq