Make error message for trying to train after make_generation_fast work correctly

Summary: https://github.com/pytorch/fairseq/blob/master/fairseq/trainer.py#L164 calls `train()` without any argument

Reviewed By: myleott

Differential Revision: D13599203

fbshipit-source-id: 3a096a6dd35a7a3f8309fbda3b54a36f606475e3

fairseq/models/fairseq_model.py

@@ -117,7 +117,7 @@ class BaseFairseqModel(nn.Module):
 
         self.apply(apply_make_generation_fast_)
 
-        def train(mode):
+        def train(mode=True):
             if mode:
                 raise RuntimeError('cannot train after make_generation_fast')