@Tony_neol / @RangerOnMars disregard the comment above. This was confirmed by the development team;
As IMU is mounted at the back of the PCB. So compared to optical frame coord system, you get IMU coord system by applying the following rotation:
--roll', '3.14159', '--pitch', '0', '--yaw', '1.5708'
I hope this is correct, but I'm still not 100% sure.