FX：按逆拓扑顺序删除无用计算

1641        # Lint the graph first to make sure its topologically sorted, otherwise1642        # DCE below will not behave as expected.1643        self.lint()16441645        def has_side_effect(node):1646            if is_impure_node is not None:1647                return is_impure_node(node)1648            return node.is_impure()16491650        # Reverse iterate so that when we remove a node, any nodes used as an1651        # input to that node have an updated user count that no longer reflects1652        # the removed node.1653        changed = False1654        for node in reversed(self.nodes):1655            if not has_side_effect(node) and len(node.users) == 0:1656                self.erase_node(node)1657                changed = True16581659        return changed