Fix randomize_rigid_body_material crash with Newton backend

[ax-anoop]

Summary

randomize_rigid_body_material crashes with AttributeError: 'ArticulationView' object has no attribute 'link_paths' when using the Newton physics backend (presets=newton).

The event term's __init__ unconditionally uses PhysX-specific APIs to compute the number of collision shapes per body:

for link_path in self.asset.root_view.link_paths[0]:
    link_physx_view = self.asset._physics_sim_view.create_rigid_body_view(link_path)
    self.num_shapes_per_body.append(link_physx_view.max_shapes)

Newton's ArticulationView does not expose link_paths, max_shapes, get_material_properties(), or set_material_properties().

Note: The underlying MuJoCo-Warp engine does support runtime friction modification via model.geom_friction — this is a well-established MuJoCo capability used in production RL pipelines. The gap is in IsaacLab's Newton ArticulationView abstraction, which does not yet map these MuJoCo fields to the get/set_material_properties API that the event system expects.

Changes

• Detect backend support by checking hasattr(self.asset.root_view, "max_shapes") before accessing material APIs
• Skip gracefully with a logger.warning when the backend lacks support
• When the backend provides asset.num_shapes_per_body directly (as Newton does), use it instead of the PhysX-specific link_paths workaround
• Guard __call__ to return early if the backend doesn't support material randomization
• PhysX behavior is completely unchanged

Reproduction

python scripts/train.py \
  --task Isaac-Velocity-Flat-Anymal-D-v0 \
  --num_envs 16 --max_iterations 1 \
  presets=newton

Crashes with:

AttributeError: 'ArticulationView' object has no attribute 'link_paths'. Did you mean: 'link_names'?

After this fix, Newton training runs successfully with the material randomization event skipped and a warning logged.

Longer-term

Full material randomization on Newton requires exposing MuJoCo-Warp's model.geom_friction (and related geom properties) through the Newton ArticulationView's get_material_properties() / set_material_properties() API. MuJoCo-Warp fully supports runtime per-environment friction modification — the gap is in the IsaacLab abstraction layer, not the physics engine.

Test plan

• Verified Newton training completes with this fix (presets=newton)
• Verified PhysX training still works unchanged (default preset)
• Run existing unit tests for randomize_rigid_body_material

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