code: def testReduceSparseVariableLength(self, prefer_unique_instance_key):
      # One device per process, 2 processes, 2 replicas in total.
      inputs = [
          IndexedSlicesValue(
              values=[[1.0]], indices=[0], dense_shape=[10, 1]
          ),
          IndexedSlicesValue(
              values=[[2.0], [3.0], [4.0]],
              indices=[0, 1, 2],
              dense_shape=[10, 1],
          ),
      ]
      expect = IndexedSlices(
          values=[[1.0], [2.0], [3.0], [4.0]],
          indices=[0, 0, 1, 2],
          dense_shape=[10, 1],
      )
      self.reduce_and_verify(
          inputs,
          expect,
          self.RunOptions(
              mode=[
                  "func_graph"
              ],  # Sparse reduce is not supported in eager.
              num_processes=2,
              reduce_op=ReduceOp.SUM,
              prefer_unique_instance_key=prefer_unique_instance_key,
          ),
      )
  
imports: import collections
  import os
  import threading
  import time
  from absl.testing import parameterized
  from tensorflow.core.protobuf import config_pb2
  from tensorflow.core.protobuf import tensorflow_server_pb2
  from tensorflow.python.distribute import cluster_resolver
  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 multi_process_runner
  from tensorflow.python.distribute import multi_worker_test_base
  from tensorflow.python.distribute import reduce_util
  from tensorflow.python.distribute import test_util
  from tensorflow.python.distribute import values
  from tensorflow.python.eager import context
  from tensorflow.python.eager import def_function
  from tensorflow.python.eager import test
  from tensorflow.python.framework import constant_op
  from tensorflow.python.framework import dtypes
  from tensorflow.python.framework import errors
  from tensorflow.python.framework import indexed_slices
  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 cond
  from tensorflow.python.ops import math_ops
  from tensorflow.python.util import nest
label_desc: Use image node_id 8 for calling the CollectiveOpsTest obj's underlying member method code with example usage: obj.testReduceSparseVariableLength(prefer_unique_instance_key) without return types