def make_input_layers(self):
        """
        Extract the ordering of the input layers.
        """
        self.input_layers = []
        if hasattr(self.model, 'input_layers'):
            input_keras_layers = self.model.input_layers[:]
            self.input_layers = [None] * len(input_keras_layers)
            for layer in self.layer_list:
                keras_layer = self.keras_layer_map[layer]
                if isinstance(keras_layer, _keras.engine.topology.InputLayer):
                    if keras_layer in input_keras_layers:
                        idx = input_keras_layers.index(keras_layer)
                        self.input_layers[idx] = layer
        elif len(self.model.inbound_nodes) <= 1:
            for ts in _to_list(self.model.input):
                # search for the InputLayer that matches this ts
                for l in self.layer_list:
                    kl = self.keras_layer_map[l]
                    if isinstance(kl, _keras.engine.topology.InputLayer) and kl.input == ts:
                        self.input_layers.append(l)
        else:
            raise ValueError("Input values cannot be identified.")
Extract the ordering of the input layers.