Fix semisupervised_translation task (#706)

Summary:
Pass required "sample_key" argument to forward-backward call in semi-supervised task.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/706

Differential Revision: D15217957

Pulled By: pipibjc

fbshipit-source-id: bf943d566c5caa67682dfb16ff8b7c432323cdba

fairseq/tasks/semisupervised_translation.py

@@ -345,7 +345,7 @@ class SemisupervisedTranslationTask(MultilingualTranslationTask):
 
         if self.lambda_parallel > 0.0:
             for lang_pair in self.args.lang_pairs:
-                forward_backward(model.models[lang_pair], sample[lang_pair], self.lambda_parallel)
+                forward_backward(model.models[lang_pair], sample[lang_pair], lang_pair, self.lambda_parallel)
 
         if self.lambda_otf_bt > 0.0:
             for lang_pair in self.args.lang_pairs: