Checks that the normalization constructor is callable.

def _check_and_assign_normalization_members(self, normalization_ctor,
                                              normalization_kwargs):
    """Checks that the normalization constructor is callable."""
    if isinstance(normalization_ctor, six.string_types):
      normalization_ctor = util.parse_string_to_constructor(normalization_ctor)
    if normalization_ctor is not None and not callable(normalization_ctor):
      raise ValueError(
          "normalization_ctor must be a callable or a string that specifies "
          "a callable, got {}.".format(normalization_ctor))
    self._normalization_ctor = normalization_ctor
    self._normalization_kwargs = normalization_kwargs