For each mesh axis, which Tensor axis maps to it.

    Args:
      mesh_ndims: int.

    Returns:
      Tuple of optional integers, with length mesh_ndims.

def mesh_axis_to_tensor_axis(self, mesh_ndims):
    """For each mesh axis, which Tensor axis maps to it.

    Args:
      mesh_ndims: int.

    Returns:
      Tuple of optional integers, with length mesh_ndims.
    """
    ta2ma = self._tensor_axis_to_mesh_axis
    return tuple(
        [ta2ma.index(mesh_axis) if mesh_axis in ta2ma else None
         for mesh_axis in xrange(mesh_ndims)])