[Feature] Support USD format

[yilin404]

As title, I wonder that how .USD file can use in sapien. I don't find a method like create_usd_loader etc.

Thanks again.

[fbxiang]

If you are using USD as a mesh format, SAPIEN 3 will try to invoke blender to convert USD into gltf and then load that. However, if you are referring to some non-standard USD such as ones generated by certain NVIDIA tools, it would include too much custom logic and it would make more sense to implement in a higher level wrapper library.