message: add support to use BackupAndRestore callback in ParameterServerStrategy.
diff: @@ -33,6 +33,7 @@ from tensorflow.python.data.ops import iterator_ops
  from tensorflow.python.distribute import collective_all_reduce_strategy
  from tensorflow.python.distribute import distribution_strategy_context as ds_context
  from tensorflow.python.distribute import mirrored_strategy
  +from tensorflow.python.distribute import parameter_server_strategy_v2
  from tensorflow.python.distribute import tpu_strategy
  from tensorflow.python.eager import context
  from tensorflow.python.framework import constant_op
  @@ -1645,7 +1646,8 @@ class BackupAndRestore(Callback):
  self._supported_strategies = (
  mirrored_strategy.MirroredStrategy,
  collective_all_reduce_strategy.CollectiveAllReduceStrategy,
  - tpu_strategy.TPUStrategy, tpu_strategy.TPUStrategyV2)
  + tpu_strategy.TPUStrategy, tpu_strategy.TPUStrategyV2,
  + parameter_server_strategy_v2.ParameterServerStrategyV2)
  if not context.executing_eagerly():
  if ops.inside_function():
  