How to write a launch file for ros_ign_bridge ?

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I recommend you look into the examples on the ros_ign_gazebo_demos package. For example, here's a snippet from diff_drive.launch.py:

# Bridge
bridge = Node(
    package='ros_ign_bridge',
    executable='parameter_bridge',
    arguments=['/model/vehicle_blue/cmd_vel@geometry_msgs/msg/Twist@ignition.msgs.Twist',
               '/model/vehicle_blue/odometry@nav_msgs/msg/Odometry@ignition.msgs.Odometry',
               '/model/vehicle_green/cmd_vel@geometry_msgs/msg/Twist@ignition.msgs.Twist',
               '/model/vehicle_green/odometry@nav_msgs/msg/Odometry@ignition.msgs.Odometry'],
    output='screen'
)