解释下面代码的意思from launch.exit_handler import ignore_exit_handler, restart_exit_handler from ros2run.api import get_executable_path def launch(launch_descriptor, argv): ld = launch_descriptor package = 'ydlidar_ros2_driver' ld.add_process( cmd=[get_executable_path(package_name=package, executable_name='ydlidar_ros2_driver_node')], name='ydlidar_ros2_driver_node', exit_handler=restart_exit_handler, ) package = 'tf2_ros' ld.add_process( # The XYZ/Quat numbers for base_link -> laser_frame are taken from the # turtlebot URDF in # https://github.com/turtlebot/turtlebot/blob/931d045/turtlebot_description/urdf/sensors/astra.urdf.xacro cmd=[distance get_executable_path( package_name=package, executable_name='static_transform_publisher'), '0', '0', '0.02', '0', '0', '0', '1', 'base_link', 'laser_frame' ], name='static_tf_pub_laser', exit_handler=restart_exit_handler, ) return ld

这段代码是一个ROS2的launch文件，用于启动两个节点，一个是ydlidar_ros2_driver_node节点，另一个是static_transform_publisher节点。其中，ydlidar_ros2_driver_node节点启动后会使用restart_exit_handler来处理它的退出，这意味着如果该节点因某种原因退出，它将被自动重启。static_transform_publisher节点也是类似的，它会通过restart_exit_handler处理退出并被自动重启。get_executable_path函数用于获取指定包中指定可执行文件的路径，这里用于获取ydlidar_ros2_driver_node和static_transform_publisher的路径。最后，launch函数返回一个launch描述符，用于启动ROS2节点。