fix bug for masking (#752)
fix bug for masking (#752)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/752

previously we sample masked tokens with replace=True (default). Because of this, we would mask same tokens multiple times, which will make us mask less tokens finally

Reviewed By: liaimi

Differential Revision: D15403556

fbshipit-source-id: cf12eeb13f9610431136a345de9199ad0292984b

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq