fix when the ensembled models do not have a decoder, e.g., FairseqEnc… (#1577)
fix when the ensembled models do not have a decoder, e.g., FairseqEnc… (#1577)

Summary:
…oderModel
Pull Request resolved: https://github.com/pytorch/fairseq/pull/1577

Differential Revision: D19289850

Pulled By: myleott

fbshipit-source-id: c4193a01e38b4e246dc60603ba8c89b938612ac2

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq