Make BART models compatiable with JIT
Make BART models compatiable with JIT

Summary: Bart models are not compatible with JIT. This diff makes minor changes to enable its compabilitity

Reviewed By: myleott

Differential Revision: D24824963

fbshipit-source-id: 41cbcc46c14b0439f5763478b8efe98e5516dc95

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq