def gate_nodes(self):
        """Get the list of gate nodes in the dag.

        Returns:
            list: the list of node ids that represent gates.
        """
        nodes = []
        for node in self.op_nodes():
            if isinstance(node.op, Gate):
                nodes.append(node)
        return nodes
Get the list of gate nodes in the dag.

        Returns:
            list: the list of node ids that represent gates.