[Proposal] Support Newton MJWarp for No-joint Scenes

[YizeWang]

Proposal

Make scripts/demos/procedural_terrain.py --color_scheme height --physics newton_mjwarp run without requiring a robot, joint, or dummy body.

Motivation

The procedural terrain demo is a static terrain inspection scene. It creates a generated terrain mesh and steps the simulator, but it does not spawn any articulation or jointed body.

Current failure:

./isaaclab.sh -p scripts/demos/procedural_terrain.py --color_scheme height --physics newton_mjwarp

Newton builds/finalizes the model, then MJWarp solver setup fails:

ValueError: The model must have at least one joint to be able to convert it to MuJoCo.

Failure path:

sim.reset()
  -> NewtonManager.initialize_solver()
  -> NewtonMJWarpManager._build_solver()
  -> SolverMuJoCo(...)
  -> _convert_to_mjc()

Root cause: MJWarp currently assumes the Newton model has at least one joint. Terrain-only scenes are valid for visualization and static scene inspection, but they have no joints.

Build Info

• Isaac Lab Version: On top of Support Multi-Back for Demo Scripts (Part 2) and Unify Code Structure #6140
• Isaac Lab Contrib Version: 0.4.4
• Isaac Lab Newton Version: 0.15.2
• Isaac Sim Version: 6.0.0-alpha.212

Acceptance Criteria

• The command above reaches Setup complete.
• Newton MJWarp supports reset/step/render for terrain-only scenes.
• No hidden dummy body is required.

[AntoineRichard]

@YizeWang could you open an issue in Newton about this? Please reference this issue in the newton issue.