def lambda_step_func(config, n_iter):
            """
            Update a lambda value according to its schedule configuration.
            """
            ranges = [
                i
                for i in range(len(config) - 1)
                if config[i][0] <= n_iter < config[i + 1][0]
            ]
            if len(ranges) == 0:
                assert n_iter >= config[-1][0]
                return config[-1][1]
            assert len(ranges) == 1
            i = ranges[0]
            x_a, y_a = config[i]
            x_b, y_b = config[i + 1]
            return y_a + (n_iter - x_a) * float(y_b - y_a) / float(x_b - x_a)

            Update a lambda value according to its schedule configuration.
            
understand_icl/fairseq/fairseq/tasks/semisupervised_translation.py