message: all_gather_v2 calls collective_gather_v2 instead of collective_gather
  which was a mistake.
diff: @@ -177,9 +177,8 @@ def all_gather_v2(t,
  Returns:
  An Op implementing the distributed operation.
  """
  - return gen_collective_ops.collective_gather(
  + return gen_collective_ops.collective_gather_v2(
  t,
  - shape=[0],
  group_size=group_size,
  group_key=group_key,
  instance_key=instance_key,
  