Fix hub decoding for GPT-2 BPE with <mask> tokens
Fix hub decoding for GPT-2 BPE with <mask> tokens

Summary: Pull Request resolved: https://github.com/pytorch/fairseq/pull/1545

Differential Revision: D19216003

Pulled By: myleott

fbshipit-source-id: 7f26963283f823eb8c5f6b13023e61c815f7b118

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq