Build a tensorflow representation of the prior density.
        The log Jacobian is included.

def _build_prior(self, unconstrained_tensor, constrained_tensor):
        """
        Build a tensorflow representation of the prior density.
        The log Jacobian is included.
        """
        if not misc.is_tensor(unconstrained_tensor):
            raise GPflowError("Unconstrained input must be a tensor.")

        if not misc.is_tensor(constrained_tensor):
            raise GPflowError("Constrained input must be a tensor.")

        prior_name = 'prior'

        if self.prior is None:
            return tf.constant(0.0, settings.float_type, name=prior_name)

        log_jacobian = self.transform.log_jacobian_tensor(unconstrained_tensor)
        logp_var = self.prior.logp(constrained_tensor)
        return tf.squeeze(tf.add(logp_var, log_jacobian, name=prior_name))