Polish code

test=develop

python/paddle/fluid/tests/unittests/test_imperative_ptb_rnn.py

@@ -16,6 +16,7 @@ from __future__ import print_function
 
 import unittest
 import paddle.fluid as fluid
+import paddle.fluid.core as core
 from paddle.fluid.dygraph.nn import Embedding
 import paddle.fluid.framework as framework
 from paddle.fluid.optimizer import SGDOptimizer