Articulation root API can not be set while simulation is playing

[hang-yin]

When you import a USD while the simulation is playing, the object gets loaded into PhysX before post_load can run, meaning it gets loaded with the articulation root API still on w/e prim it was on in the USD regardless of whether the USD is fixed or not, and you can't fix it without stopping (e.g. even if you move the articulation root API on the USD, physx remains unaware).
We can hack this by removing the articulation root API from all of our USDs, then at load time we can pre-load it using the Usd library instead of loading it directly into Isaac, add the articulation root, and load that one into Isaac instead.

#819 (comment)

[cgokmen]

@ChengshuLi FYI