docs: def _partition_spec(self, shape, partition_info):
      """Build magic (and sparsely documented) shapes_and_slices spec string."""
      if partition_info is None:
        return ''  # Empty string indicates a non-partitioned tensor.
      ssi = tf.Variable.SaveSliceInfo(
          full_name=self._var_name,
          full_shape=partition_info.full_shape,
          var_offset=partition_info.var_offset,
          var_shape=shape)
      return ssi.spec