add missing function to FairseqLanguageModel

Summary: In https://github.com/fairinternal/fairseq-py/pull/877, sequence_generator began calling `model.forward_decoder`, but not all decoder models were given an implementation of that function.

Reviewed By: okhonko

Differential Revision: D17863751

fbshipit-source-id: ea70b636c9dafcf87f5d5e49631d0c4b7cf14984

fairseq/models/fairseq_model.py

@@ -369,6 +369,9 @@ class FairseqLanguageModel(BaseFairseqModel):
         """
         return self.decoder(src_tokens, **kwargs)
 
+    def forward_decoder(self, prev_output_tokens, **kwargs):
+        return self.decoder(prev_output_tokens, **kwargs)
+
     def extract_features(self, src_tokens, **kwargs):
         """
         Similar to *forward* but only return features.