Revert "Specify NCCL as the all reduce algorithm (#6662)" (#6671)

Reason: test failures because contrib is not available in V2

This reverts commit 325dd761.

official/resnet/keras/keras_imagenet_main.py

@@ -124,8 +124,7 @@ def run(flags_obj):
   strategy = distribution_utils.get_distribution_strategy(
       distribution_strategy=flags_obj.distribution_strategy,
       num_gpus=flags_obj.num_gpus,
-      num_workers=distribution_utils.configure_cluster(),
-      all_reduce_alg='nccl')
+      num_workers=distribution_utils.configure_cluster())
 
   strategy_scope = distribution_utils.get_strategy_scope(strategy)