added missing cmd arg for bart cnn ft
added missing cmd arg for bart cnn ft

Summary: Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/930

Differential Revision: D18763707

fbshipit-source-id: 453a877f5bb39c5afcf7f9bc101019b1be4a0a60

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq