[FIX] Defer asset imports until Kit startup

[mmichelis]

Description

Defer the cable and deformable runtime asset imports until after launch_simulation() starts Kit. This prevents the standalone USD runtime from loading before Kit initializes its bundled USD runtime. The type annotations are kept via TYPE_CHECKING imports, matching the existing pattern in scripts/demos/multi_asset.py and scripts/demos/procedural_terrain.py.

Correct the documented commands for both demos so they install the extras their default configuration actually needs. The cable demo defaults to the Kit visualizer, so it needs --extra isaacsim. The deformable tutorial defaults to Kit and always tetrahedralizes its volume deformables, so it needs --extra isaacsim --extra tetrahedralization; its OvPhysX example stays Kitless but needs --extra ovphysx --extra tetrahedralization. The cable examples that pass --visualizer newton or --visualizer none are intentionally left Kitless.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Documentation update

Screenshots

Not applicable.

Validation

• uv run --frozen isaaclab -f
• Cable demo completed two steps with the Kit visualizer.
• Deformable-object tutorial started Kit and simulated with each of the three backends, using the documented commands.
• Confirmed each documented extra is genuinely required: with the tetrahedralization dependencies unavailable, all three backends fail in define_deformable_body_properties.
• Documentation build completed successfully.

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks
• I have made corresponding documentation changes
• My changes generate no new warnings
• No source-package changelog fragment is required
• My name already exists in CONTRIBUTORS.md

[greptile-apps]

Greptile Summary

This PR defers cable and deformable asset imports until after simulation runtime initialization and updates the default cable commands to install the Kit visualizer dependencies.

• Moves runtime asset imports into each script’s scene-construction function.
• Preserves static type-checking imports with TYPE_CHECKING.
• Adds the isaacsim extra to documented default cable invocations.

Confidence Score: 5/5

The PR appears safe to merge with no actionable defects identified.

The deferred imports occur after runtime initialization, type-only references remain safe, and the updated commands use the repository’s defined Isaac Sim extra only for the Kit-dependent path.

Important Files Changed

Filename	Overview
scripts/demos/cables.py	Defers cable asset imports until scene construction, after runtime launch and simulation-context initialization.
scripts/tutorials/01_assets/run_deformable_object.py	Defers deformable asset imports safely; the remaining local variable annotation does not require runtime name resolution.
docs/source/overview/core-concepts/physical-backends/newton/using-cables.rst	Adds the valid isaacsim extra to the default Kit-based cable command while preserving Kitless examples.
docs/source/overview/showroom.rst	Keeps the showroom cable command consistent with the default Kit visualizer dependency requirement.

_{Reviews (1): Last reviewed commit: "Fix Kit startup for cable and deformable..." | Re-trigger Greptile}

[isaaclab-review-bot]

Isaac Lab Review Bot

The change defers cable and deformable asset imports until scene construction, avoiding their runtime initialization during pre-Kit module loading. The default cable documentation now installs the isaacsim extra required by the Kit visualizer.

• Design and architecture: The lazy imports are narrowly scoped to design_scene() in both scripts, while CLI parsing and launcher setup remain at module scope. This preserves the existing script structure and limits the change to the asset imports implicated in startup ordering.
• API: Public APIs and CLI options remain unchanged. The documented default cable invocation now matches its Kit visualizer dependency, while explicitly Kitless visualizer examples remain unchanged.
• Implementation: The asset classes and configurations are imported at scene-construction time, with type-only imports retained under TYPE_CHECKING. Postponed annotation evaluation protects the cable demo’s CableObject annotations, and the shown deformable tutorial change introduces no runtime-evaluated annotation requiring the deferred symbol.

No blocking issues. No inline issue met the actionable-evidence threshold; the assessment above records the review feedback.

Automated review; human maintainers own approval decisions.