Add a layer to the Graph.
        Args:
            layer: An instance of the subclasses of StubLayer in layers.py.
            input_node_id: An integer. The ID of the input node of the layer.
        Returns:
            output_node_id: An integer. The ID of the output node of the layer.

def add_layer(self, layer, input_node_id):
        """Add a layer to the Graph.
        Args:
            layer: An instance of the subclasses of StubLayer in layers.py.
            input_node_id: An integer. The ID of the input node of the layer.
        Returns:
            output_node_id: An integer. The ID of the output node of the layer.
        """
        if isinstance(input_node_id, Iterable):
            layer.input = list(map(lambda x: self.node_list[x], input_node_id))
            output_node_id = self._add_node(Node(layer.output_shape))
            for node_id in input_node_id:
                self._add_edge(layer, node_id, output_node_id)

        else:
            layer.input = self.node_list[input_node_id]
            output_node_id = self._add_node(Node(layer.output_shape))
            self._add_edge(layer, input_node_id, output_node_id)

        layer.output = self.node_list[output_node_id]
        return output_node_id