Set proper device for MHA padding mask (#1013)
Set proper device for MHA padding mask (#1013)

Summary:
Fixes GPU training
Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/1013

Differential Revision: D19663284

Pulled By: myleott

fbshipit-source-id: ed9ec40e5abba2e4b1df5718f25376c4df5de1fb

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq