Skip RTX scene partitioning without spectator-view support

[mataylor-nvidia]

Description

Follow-up to isaac-sim#7053. On that branch the Kit viewport renders completely black for any
camera task when running against the Isaac Sim version this repo's pyproject.toml pins
(isaacsim==6.0.1.0).

_apply_viewport_camera_scene_partition removes the viewport camera's omni:scenePartition
token whenever /rtx/scenePartitioning/showAllPartitionsByDefault reads back as true, relying
on RTX to then show the camera every partition. Carb settings are schemaless, so the experience
files create that key on every runtime and reading it back cannot distinguish a renderer that
implements the spectator view from one that ignores it. The check passes on Isaac Sim 6.0, the
token is stripped, and nothing implements the fallback — so the camera matches no partition and
the viewport renders nothing.

Reproduced with 4 environments and the Isaac RTX renderer:

HEADLESS=1 python scripts/reinforcement_learning/train.py --rl_library rsl_rl \
  --task Isaac-Cartpole-Camera --num_envs 4 --video --video_length 60 \
  --max_iterations 2 --viz kit \
  env.viewer.eye=[38.0,38.0,26.0] env.viewer.lookat=[0.0,0.0,0.0]

Build	Non-black pixels	Envs visible
develop, partitioning on	736	1 / 4
isaac-sim#7053, partitioning on	0	0 / 4
isaac-sim#7053, partitioning off (control)	3962	4 / 4
isaac-sim#7053 + this change	3962	4 / 4

Confirmed against the installed runtime: /renderer/scenePartitioning/enabled appears in
rtx.scenedb.plugin.dll and carb.scenerenderer-rtx.plugin.dll, while no showAllPartitions
string exists in any RTX binary shipped with Isaac Sim 6.0.0-rc.59 (Kit 6312fa25).

Changes

• Add show_all_partitions_supported() resolving the capability from the Isaac Sim version
  (SHOW_ALL_PARTITIONS_MIN_ISAAC_SIM_VERSION = 6.1).
• IsaacRtxRenderer.prepare_stage now skips scene-partition authoring when the spectator view
  is requested but unsupported, and logs an actionable warning, rather than half-applying the
  feature. The existing visualizer guard then leaves the viewport camera untouched.
• Document the Isaac Sim requirement and the fallback in the renderers guide.

Behavior on Isaac Sim 6.1+ is unchanged. Setting
IsaacRtxRendererCfg.global_settings.show_all_partitions_by_default=False keeps per-environment
isolation on older runtimes, with the viewport bound to a single environment.

Trade-off

On Isaac Sim 6.0 this trades tiled-camera partition isolation for a working viewport. Isolation
does work on 6.0 when the spectator view is not requested, so the alternative is to keep
partitioning and have the visualizer fall back to tagging env_0 — a single-environment viewport
instead of an empty one. Happy to switch if that is preferred.

Note on the Isaac Sim pin

isaac-sim#7053 bumps only the CI container to 6.1.0-alpha.50; pyproject.toml still pins
isaacsim[all,extscache]==6.0.1.0. Until those move together, anyone installing from the wheels
rather than the CI image is on a runtime without the spectator view. This change makes that
configuration degrade to the pre-isaac-sim/IsaacLab#7053 behavior instead of an empty viewport.

Type of change

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

Screenshots

Before (isaac-sim#7053)	After (this change)
0 of 4 environments — viewport fully black	4 of 4 environments

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added a changelog fragment under source/<pkg>/changelog.d/ for every touched package (do not edit CHANGELOG.rst or bump extension.toml — CI handles that)
• I have added my name to the CONTRIBUTORS.md or my name already exists there

Testing

• New test_partitioning_skipped_when_spectator_view_unsupported fails without the gate and
  passes with it; test_partitioning_kept_without_spectator_view_on_unsupported_runtime covers
  the opt-out path.
• The two existing default-behavior tests now pin the capability so they no longer depend on the
  Isaac Sim build running them.
• source/isaaclab_physx/test/renderers/ — 36 passed.
• test_kit_visualizer_scene_partitioning.py + test_renderers.py — 10 passed.

test_partitioning_isolates_rigid_object fails on Isaac Sim 6.0 both with and without this
change (byte-identical pixel matrix) — the per-instance PointInstancer marker tokens added in
isaac-sim#7053 do not isolate on that runtime either. Untouched here.

[mataylor-nvidia]

Superseded: keeping show_all_partitions_by_default=True unconditionally, with no version-gated fallback for older Isaac Sim releases.