l_fn(x, s)
            if model_s1 is None:
                x_s1 = (
                        expand_dims(sigma_s1 / sigma_s, dims) * x
                        - expand_dims(alpha_s1 * phi_11, dims) * model_s
                )
                model_s1 = self.model_fn(x_s1, s1)
            x_s2 = (
                    expand_dims(sigma_s2 / sigma_s, dims) * x
                    - expand_dims(alpha_s2 * phi_12, dims) * model_s
                    + r2 / r1 * expand_dims(alpha_s2 * phi_22, dims) * (model_s1 - model_s)
            )
            model_s2 = self.model_fn(x_s2, s2)
            if solver_type == 'dpm_solver':
                x_t = (
                        expand_dims(sigma_t / sigma_s, dims) * x
                        - expand_dims(alpha_t * phi_1, dims) * model_s
                        + (1. / r2) * expand_dims(alpha_t * phi_2, dims) * (model_s2 - model_s)
                )
            elif solver_type == 'taylor':
                D1_0 = (1. / r1) * (model_s1 - model_s)
                D1_1 = (1. / r2) * (model_s2 - model_s)
                D1 = (r2 * D1_0 - r1 * D1_1) / (r2 - r1)
                D2 = 2. * (D1_1 - D1_0) / (r2 - r1)
                x_t = (
                        expand_dims(sigma_t / sigma_s, dims) * x
                        - expand_dims(alpha_t * phi_1, dims) * model_s
                        + expand_dims(alpha_t * phi_2, dims) * D1
                        - expand_dims(alpha_t * phi_3, dims) * D2
                )