code: def testChooseAlgorithm(self):
      # Not use nccl if there is any cpu device.
      self.assertIsInstance(
          cross_device_ops_lib.select_cross_device_ops(["/cpu:0"]),
          cross_device_ops_lib.ReductionToOneDevice,
      )
  
      # Not use nccl if requested device is not visible to TensorFlow.
      # TODO(yuefengz): make `select_cross_device_ops` work with device strings
      # self.assertIsInstance(
      #     cross_device_ops_lib.select_cross_device_ops(["/gpu:100"]),
      #     cross_device_ops_lib.ReductionToOneDevice)
  
      if context.num_gpus() < 1:
          return
  
      devices = ["/gpu:0"]
  
      def mock_get_registered_kernels_for_op(op):
          if op == "NcclAllReduce":
              return [object]
          else:
              return []
  
      # Use nccl if nccl kernel is found.
      with test.mock.patch.object(
          kernels,
          "get_registered_kernels_for_op",
          mock_get_registered_kernels_for_op,
      ):
          self.assertIsInstance(
              cross_device_ops_lib.select_cross_device_ops(devices),
              cross_device_ops_lib.NcclAllReduce,
          )
  
      # Not use nccl if nccl kernel is not found.
      with test.mock.patch.object(
          kernels, "get_registered_kernels_for_op", lambda _: []
      ):
          self.assertIsInstance(
              cross_device_ops_lib.select_cross_device_ops(devices),
              cross_device_ops_lib.ReductionToOneDevice,
          )
  
imports: import itertools
  import os
  import threading
  import time
  from absl.testing import parameterized
  import numpy
  from tensorflow.core.protobuf import config_pb2
  from tensorflow.python.distribute import cluster_resolver
  from tensorflow.python.distribute import collective_all_reduce_strategy
  from tensorflow.python.distribute import collective_util
  from tensorflow.python.distribute import combinations
  from tensorflow.python.distribute import cross_device_ops
  from tensorflow.python.distribute import cross_device_utils
  from tensorflow.python.distribute import device_util
  from tensorflow.python.distribute import distribute_utils
  from tensorflow.python.distribute import multi_worker_test_base
  from tensorflow.python.distribute import multi_worker_util
  from tensorflow.python.distribute import reduce_util
  from tensorflow.python.distribute import strategy_combinations
  from tensorflow.python.distribute import values
  from tensorflow.python.eager import context
  from tensorflow.python.eager import test
  from tensorflow.python.framework import constant_op
  from tensorflow.python.framework import indexed_slices
  from tensorflow.python.framework import kernels
  from tensorflow.python.framework import ops
  from tensorflow.python.framework import tensor
  from tensorflow.python.ops import array_ops
  from tensorflow.python.ops import collective_ops
  from tensorflow.python.ops import math_ops
  from tensorflow.python.ops import variables
label_desc: Use image node_id 2 for calling the SingleWorkerCrossDeviceOpsTest obj's underlying member method code with example usage: obj.testChooseAlgorithm() and returns: list, list