Small fix for trainer

Summary: Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/711

Differential Revision: D16192752

Pulled By: myleott

fbshipit-source-id: 102ed337a3d31e2047be7c033e9007c04223a684

fairseq/trainer.py

@@ -158,7 +158,7 @@ class Trainer(object):
 
             extra_state = state['extra_state']
             self._optim_history = state['optimizer_history']
-            last_optim_state = state['last_optimizer_state']
+            last_optim_state = state.get('last_optimizer_state', None)
 
         if last_optim_state is not None and not reset_optimizer:
             # rebuild optimizer after loading model, since params may have changed