explicitly use nccl

bob/learn/tensorflow/configs/MultiWorkerMirroredStrategy.py

-import logging
-
 import tensorflow as tf
 
-logger = logging.getLogger(__name__)
-
 
 def strategy_fn():
-    print("creating strategy")
-    strategy = tf.distribute.MultiWorkerMirroredStrategy()
-    print("strategy created")
+    print("Creating MultiWorkerMirroredStrategy strategy.")
+    strategy = tf.distribute.MultiWorkerMirroredStrategy(
+        communication_options=tf.distribute.experimental.CommunicationOptions(
+            implementation=tf.distribute.experimental.CollectiveCommunication.NCCL
+        )
+    )
+    print("MultiWorkerMirroredStrategy strategy created.")
     return strategy