Fix transformer LM arg upgrade logic (#1717)
Fix transformer LM arg upgrade logic (#1717)

Summary: Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/1717

Reviewed By: alexeib

Differential Revision: D27156919

Pulled By: myleott

fbshipit-source-id: af3c2e41464c04a7808f40894e7b0106798e4822

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq