Build a tf expression for the prior by summing all child-parameter priors.

def _build_prior(self, prior_tensors):
        """
        Build a tf expression for the prior by summing all child-parameter priors.
        """
        # TODO(@awav): What prior must represent empty list of parameters?
        if not prior_tensors:
            return tf.constant(0, dtype=settings.float_type)
        return tf.add_n(prior_tensors, name='prior')