fix optim history

fairseq/trainer.py

@@ -61,8 +61,9 @@ class Trainer(object):
     def save_checkpoint(self, filename, extra_state):
         """Save all training state in a checkpoint file."""
         if distributed_utils.is_master(self.args):  # only save one checkpoint
+            optim_history = self._optim_history if hasattr(self, '_optim_history') else None
             utils.save_state(filename, self.args, self.model, self.criterion, self.optimizer,
-                             self.lr_scheduler, self._num_updates, self._optim_history, extra_state)
+                             self.lr_scheduler, self._num_updates, optim_history, extra_state)
 
     def load_checkpoint(self, filename):
         """Load all training state from a checkpoint file."""