message: Allow TPUMirroredVariable to be a valid reduction destination
diff: @@ -53,7 +53,8 @@ def validate_destinations(destinations):
  if not isinstance(
  destinations,
  (value_lib.DistributedValues, resource_variable_ops.ResourceVariable,
  - value_lib.AggregatingVariable, six.string_types, list, tuple)):
  + value_lib.AggregatingVariable, six.string_types, list, tuple,
  + value_lib.TPUMirroredVariable)):
  raise ValueError("destinations must be one of a `DistributedValues` object,"
  " a tf.Variable object, a device string, a list or tuple "
  "of device strings")
  