fix bug on exporting

src/sdk/pynni/nni/nas/pytorch/pdarts/trainer.py

 # Copyright (c) Microsoft Corporation.
 # Licensed under the MIT license.
 import logging
+
 from nni.nas.pytorch.callbacks import LRSchedulerCallback
 from nni.nas.pytorch.darts import DartsTrainer
 from nni.nas.pytorch.trainer import BaseTrainer
@@ -66,8 +67,5 @@ class PdartsTrainer(BaseTrainer):
     def validate(self):
         self.model.validate()
 
-    def export(self):
-        self.mutator.export()
-
     def checkpoint(self):
         raise NotImplementedError("Not implemented yet")