added single node distdataparallel

pytorch_lightning/models/trainer.py

@@ -292,6 +292,7 @@ class Trainer(TrainerIO):
     def __getstate__(self):
         """ This is called before pickling. """
         state = self.__dict__.copy()
+        del state['cluster']
         return {}
 
     def __dp_train(self, gpu_nb, proc_rank):