Add Newton USD Schemas as a dependency for MjcPhysics Schemas

[andrewkaufman]

Newton USD Schemas provide extensions to OpenUSD's UsdPhysics specification, focussed on robotics use cases. As Newton supports MuJoCo simulation (via mujoco_warp), many of these attributes are directly applicable to MjcPhysics.

Here we deprecate equivalent attributes in MjcPhysics and apply the corresponding Newton schemas as built-ins. By unifying the USD attributes between Newton & MjcPhysics, we provide a more straightforward USD authoring experience for consumers.

There is no official USD schema deprecation mechanism today, so instead we apply the following changes:

• In the schema files, we markup all associated doc strings as deprecated.
• In the usd_decoder we parse both attributes. When a deprecated mjc attr is authored it takes precedence, but we emit a warning indicating the user should migrate to the newton attribute.
• In the mujoco_to_usd writer we only author the newton attributes & skip authoring for the deprecated mjc attributes.

There are also a couple semi-controvertial changes here that I want to callout for visibility:

• The entire MjcMaterialAPI is now deprecated. Since NewtonMaterialAPI provides the exact same attributes (albeit with different fallback values) I didn't see a reason to retain the schema
• The margin/gap semantics currently differ between newton & mujoco
  • newton:contactMargin = mjc:margin - mjc:gap
  • newton:contactGap = mjc:gap
  • My understanding is this will be aligned in a future release, but it seemed best to be truthful to what is happening now & adapt when that change occurs
• MjcEqualityJointAPI now prepends NewtonMimicAPI rather than MjcEqualityAPI
  • The necessary attributes from MjcEqualityAPI are replicated on MjcEqualityJointAPI, excluding mjc:target since that is redundant with newton:mimicJoint
  • This frames MjcEqualityJointAPI more as a higher order extension of mimics, rather than as another mujoco equality.
  • I can see how that is perhaps a bit odd from a mujoco user's perspective, but when coef2-4 are 0 or unauthored, it is an exact mimic.

Build Issues

Newton USD Schemas are a codeless schema, which makes the build dependency fairly trivial, just downloading the schemas from GitHub & installing them. Unfortunately, this benefit comes with a drawback in that we need to hardcode the schema tokens in the mujoco cc files.

Additionally, there is no way currently for an external to run the necessary usdGenSchema and clang formatting calls needed when editing the MjcPhyiscs schemas. That will need to be done internally after this is accepted.

I'd recommend also adding a proper C++ [[deprecated]] (or mujoco equivalent) decorator on the generated schema attributes if possible. It will be much more discoverable for folks that way.