message: Add support for TPUStrategy to be able to unwrap TPUMirroredVariable.
diff: @@ -521,6 +521,11 @@ class TPUExtended(distribute_lib.DistributionStrategyExtended):
  # be represented using a PerReplica wrapper instead of a list with
  # one entry per device.
  return tuple(val)
  + elif isinstance(val, values.TPUMirroredVariable):
  + # pylint: disable=protected-access
  + if values._enclosing_tpu_context() is not None:
  + return (val,)
  + return tuple(val._get(device=d) for d in sorted(val._index.keys()))
  return (val,)
  def value_container(self, value):
  