GRUBlockCell should be imported

registeration work is done when importing, otherwise when we restore graph
from pb file and the graph contains GRUBlockCell then it will fail to run.

Author: zhijxu-MS

tests/run_pretrained_models.py

@@ -30,6 +30,7 @@
 from tf2onnx import utils
 from tf2onnx.graph import GraphUtil
 from tf2onnx.tfonnx import process_tf_graph
+from tensorflow.contrib.rnn import GRUBlockCell  # pylint: disable=unused-import
 
 # pylint: disable=broad-except,logging-not-lazy,unused-argument,unnecessary-lambda