message: Migrate some of combinations.py's dependency to core from contrib.
diff: @@ -45,23 +45,23 @@ import types
  import unittest
  from absl.testing import parameterized
  import six
  -
  -from tensorflow.contrib import cluster_resolver
  from tensorflow.contrib.distribute.python import mirrored_strategy as mirrored_lib
  -from tensorflow.contrib.distribute.python import one_device_strategy as one_device_lib
  from tensorflow.contrib.distribute.python import parameter_server_strategy
  -from tensorflow.contrib.distribute.python import tpu_strategy as tpu_lib
  from tensorflow.contrib.optimizer_v2 import adagrad as adagrad_v2
  from tensorflow.contrib.optimizer_v2 import adam as adam_v2
  from tensorflow.contrib.optimizer_v2 import gradient_descent as gradient_descent_v2
  -from tensorflow.contrib.tpu.python.tpu import device_assignment as device_assignment_lib
  from tensorflow.python.distribute import distribution_strategy_context
  +from tensorflow.python.distribute import one_device_strategy as one_device_lib
  +from tensorflow.python.distribute import tpu_strategy as tpu_lib
  +from tensorflow.python.distribute.cluster_resolver import tpu_cluster_resolver
  from tensorflow.python.eager import context
  from tensorflow.python.framework import ops
  from tensorflow.python.keras.optimizer_v2 import adagrad as adagrad_keras_v2
  from tensorflow.python.keras.optimizer_v2 import adam as adam_keras_v2
  from tensorflow.python.keras.optimizer_v2 import gradient_descent as gradient_descent_keras_v2
  from tensorflow.python.keras.optimizer_v2 import rmsprop as rmsprop_keras_v2
  +from tensorflow.python.tpu import device_assignment as device_assignment_lib
  +from tensorflow.python.tpu import tpu_strategy_util
  from tensorflow.python.training import adagrad
  from tensorflow.python.training import adam
  from tensorflow.python.training import gradient_descent
  @@ -329,8 +329,8 @@ class NamedDistribution(object):
  def _get_tpu_strategy_creator(steps_per_run, use_single_core=False, **kwargs):
  def _create_tpu_strategy():
  - resolver = cluster_resolver.TPUClusterResolver("")
  - topology = tpu_lib.initialize_tpu_system(resolver)
  + resolver = tpu_cluster_resolver.TPUClusterResolver("")
  + topology = tpu_strategy_util.initialize_tpu_system(resolver)
  device_assignment = None
  if use_single_core:
  device_assignment = device_assignment_lib.DeviceAssignment(
  