message: Update contrib/all_reduce/python/all_reduce.py in consequence of
  earlier change to nccl.broadcast function signature.
diff: @@ -744,12 +744,12 @@ def _build_nccl_hybrid(input_tensors, red_op, upper_level_f):
  level_2_output = upper_level_f(up_values)
  # Third stage: propagate within each worker using NCCL Broadcast
  for w in range(0, num_workers):
  - dst_devices = per_worker_devices[w][1:]
  - send_op, dst_tensors = nccl.broadcast(level_2_output[w], dst_devices)
  - # NOTE: need control dependency to ensure send_op executes
  - with ops.control_dependencies([send_op]):
  + dst_tensors = []
  with ops.device(per_worker_devices[w][0]):
  - dst_tensors.insert(0, array_ops.identity(level_2_output[w]))
  + broadcast_src = nccl.broadcast(array_ops.identity(level_2_output[w]))
  + for d in per_worker_devices[w]:
  + with ops.device(d):
  + dst_tensors.append(array_ops.identity(broadcast_src))
  down_values[w] = dst_tensors
  output_tensors = [v for sublist in down_values for v in sublist]
  if len(shape) > 1:
  