Fix score reference when using translation_from_pretrained_bart task
Fix score reference when using translation_from_pretrained_bart task

Summary: `translation_from_pretrained_bart` task doesn't have `target_lang` variable. We need to access it through `args` like line 104.

Reviewed By: myleott

Differential Revision: D21369854

fbshipit-source-id: a405eb6525d2484c2a7a91e0db47def8ae203d30

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq