Allow attaching to ArticulationObjCfg

[dementrock]

For example, attaching hands to a robot's wrists, attaching camera to a robot's head, etc. So that one doesn't need to create separate combinations of usd/mjcf/urdf files just for different hand mounting.

[Fisher-Wang]

Thank you Rocky, this is a good idea!

Practically speaking, I think this could be more straightforward to implement for mjcf and urdf, but not for usd. For usd we have to use OpenUSD API instead of simply parsing and writing xml. Plus, simply combine different part of robot into a usd doesn't necessarily mean they are successfully assembled to be able to be controlled as a whole. As shown in the official tutorial, the assembly process can be quite complicated, and finding an OpenUSD script equivalent may take some time.

Reference:

• Assembly ur5e and robotiq 2f85 mjcf: Attaching the RobotIQ 2f85 gripper to the UR10e robotic arm. google-deepmind/mujoco_menagerie#37 (comment)

[dementrock]

Thanks @Fisher-Wang ! If we support #45, perhaps not having the usd support is ok?

[Fisher-Wang]

Yeah this should be relatively easy to do