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

fairseq/tasks/translation_from_pretrained_bart.py

@@ -84,7 +84,7 @@ def build_generator(self, models, args):
             from fairseq.sequence_scorer import SequenceScorer
             return SequenceScorer(
                 self.target_dictionary,
-                eos=self.tgt_dict.index('[{}]'.format(self.target_lang))
+                eos=self.tgt_dict.index('[{}]'.format(self.args.target_lang))
             )
         else:
             from fairseq.sequence_generator import SequenceGenerator