Change `CollectiveAllReduceStrategy` to `MultiWorkerMirroredStrategy`. (#6282)

* s/CollectiveAllReduceStrategy/MultiWorkerMirroredStrategy

* More s/contrib.distribute/distribute.experimental

official/utils/misc/distribution_utils.py

@@ -63,8 +63,7 @@ def get_distribution_strategy(distribution_strategy="default",
     return None
 
   if distribution_strategy == "multi_worker_mirrored" or num_workers > 1:
-    return tf.contrib.distribute.CollectiveAllReduceStrategy(
-        num_gpus_per_worker=num_gpus)
+    return tf.distribute.experimental.MultiWorkerMirroredStrategy()
 
   if (distribution_strategy == "one_device" or
       (distribution_strategy == "default" and num_gpus <= 1)):
@@ -91,8 +90,7 @@ def get_distribution_strategy(distribution_strategy="default",
       return tf.distribute.MirroredStrategy(devices=devices)
 
   if distribution_strategy == "parameter_server":
-    return tf.contrib.distribute.ParameterServerStrategy(
-        num_gpus_per_worker=num_gpus)
+    return tf.distribute.experimental.ParameterServerStrategy()
 
   raise ValueError(
       "Unrecognized Distribution Strategy: %r" % distribution_strategy)