Fix EMA GPU test
Fix EMA GPU test

Summary: The GPU test was broken after D33809223 (https://github.com/pytorch/fairseq/commit/1b61bbad327d2bf32502b3b9a770b57714cc43dc)

Reviewed By: cruvadom

Differential Revision: D33931570

fbshipit-source-id: 37962a437d8e25b1dafc58db0efa55c1afa5f3ee

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq