Get the min mu which minimize the surrogate.

    Returns:
      The mu_t.

def _get_mu_tensor(self):
    """Get the min mu which minimize the surrogate.

    Returns:
      The mu_t.
    """
    root = self._get_cubic_root()
    dr = self._h_max / self._h_min
    mu = tf.maximum(
        root**2, ((tf.sqrt(dr) - 1) / (tf.sqrt(dr) + 1))**2)
    return mu