Add flat mesh slots for heterogeneous multi-mesh raycasts

[ooctipus]

Motivation

PR #5510 adds ClonePlan-based multi-mesh target discovery as part of the RayCaster backend split. To keep that PR conservative, heterogeneous layouts still use the existing rectangular dynamic-mesh kernel with dummy padding for environments that have fewer meshes.

This PR removes that padding cost. Instead of launching raycasts over num_envs * max_meshes_per_env, it launches over the actual flat list of real mesh targets.

What Changed

• Added raycast_dynamic_mesh_slots_kernel, launched as (num_slots, num_rays).
• Switched MultiMeshRayCaster and MultiMeshRayCasterCamera from rectangular padded tables to flat mesh slots.
• Added slot_env_ids so each mesh slot maps back to its owning environment.
• Updated PhysX and Newton dynamic target pose refresh to write into flat slot buffers.
• Updated ClonePlan integration coverage to assert heterogeneous slot counts [1, 2, 1] without USD object clones.

Why

The rectangular kernel is correct, but for heterogeneous ClonePlan layouts it does work for dummy meshes. Flat slots match the actual scene layout: one slot per real mesh, no fake mesh ids, no far-away padding poses, and less wasted kernel work.

Dependency

Depends on #5510. Please review and land #5510 first, then review this PR as the follow-up flat-slot optimization. After #5510 lands, this PR should be rebased onto develop so the diff only shows the flat-slot change.

Test Plan

• ./isaaclab.sh -p -m pytest source/isaaclab/test/sensors/test_ray_caster_integration.py::test_multi_mesh_consumes_clone_plan_without_usd_object_clones -q
• ./isaaclab.sh -p -m pytest source/isaaclab/test/sensors/test_multi_mesh_ray_caster_camera.py -q
• ./isaaclab.sh -p -m pytest source/isaaclab/test/sensors/test_ray_caster_integration.py::test_update_mesh_transforms_non_identity_offset -q
• ./isaaclab.sh -f

[greptile-apps]

Greptile Summary

This PR replaces the rectangular padded-table layout for multi-mesh raycasts with a compact flat-slot Warp kernel (raycast_dynamic_mesh_slots_kernel), enabling heterogeneous per-environment mesh counts without padding. It also restructures ClonePlan from a per-group object (with dest_template/prototype_paths) into a single flat plan (sources/destinations/clone_mask), moves heterogeneous clone-plan construction into InteractiveScene, and extracts shared ray-caster logic into backend-agnostic base classes.

• New raycast_dynamic_mesh_slots_kernel dispatches over a flat (num_slots, num_rays) grid; each slot carries its owning environment id, eliminating per-env padding.
• ClonePlan is refactored to carry flat parallel sources/destinations lists; clone_from_template is removed and make_clone_plan now returns a ClonePlan directly.
• BaseMultiMeshRayCaster and BaseRayCaster are extracted as backend-neutral base classes; PhysX and Newton subclasses are thin mixins.

Confidence Score: 3/5

Not safe to merge as-is: two files carry stale references to removed ClonePlan fields that will raise AttributeError at runtime, and the ray_mesh_id semantic change in the new flat-slot kernel may silently corrupt per-env mesh-id lookups in downstream consumers.

The _target_tracker_utils.py module references plan.dest_template — a field that was deleted in this PR — meaning any caller of walk_target_prototypes crashes immediately. The check_multi_mesh_ray_caster.py test helper similarly constructs a ClonePlan with the old field names and calls the renamed set_clone_plans method, so the script fails before exercising any sensor logic. Additionally, the new raycast_dynamic_mesh_slots_kernel records the global slot_id in ray_mesh_id rather than the per-env mesh index used by the existing rectangular kernel, which can silently produce wrong indices for any code that treats the output as a dense per-env mesh selector.

_target_tracker_utils.py (stale dest_template access), check_multi_mesh_ray_caster.py (old ClonePlan constructor and set_clone_plans call), and kernels.py (undocumented ray_mesh_id semantic change in the flat-slot kernel).

Important Files Changed

Filename	Overview
source/isaaclab/isaaclab/utils/warp/kernels.py	Adds raycast_dynamic_mesh_slots_kernel — flat slot layout for heterogeneous scenes; ray_mesh_id stores global slot index (not per-env mesh index), diverging from the existing rectangular-kernel convention without documentation.
source/isaaclab/isaaclab/sensors/ray_caster/_target_tracker_utils.py	New shared target-tracker utility; _find_covering_plan accesses plan.dest_template — a field that no longer exists on the refactored ClonePlan, causing AttributeError at runtime for any caller.
source/isaaclab/isaaclab/sensors/ray_caster/base_multi_mesh_ray_caster.py	New base class for multi-mesh ray-casting using compact flat slots; ClonePlan integration and mesh slot initialization look structurally correct.
source/isaaclab/isaaclab/cloner/clone_plan.py	Replaces dest_template/prototype_paths fields with flat sources/destinations lists; adds eq=False to keep hash identity-based. Several consumers were not fully updated.
source/isaaclab/isaaclab/scene/interactive_scene.py	Migrates heterogeneous clone-plan construction into the scene itself; correctly uses the new flat ClonePlan API and publishes via set_clone_plan (singular).
source/isaaclab/test/sensors/check_multi_mesh_ray_caster.py	Check script uses old set_clone_plans (plural) and constructs ClonePlan with removed fields dest_template/prototype_paths; will fail immediately on execution.
source/isaaclab/isaaclab/cloner/cloner_utils.py	Removes clone_from_template; make_clone_plan now returns a ClonePlan directly instead of a tuple — clean simplification.
source/isaaclab_physx/isaaclab_physx/scene_data_providers/physx_scene_data_provider.py	Correctly migrated to flat ClonePlan; reads plan.sources/plan.destinations directly instead of reconstructing paths from dest_template.

Sequence Diagram

sequenceDiagram
    participant Scene as InteractiveScene
    participant Cloner as cloner_utils
    participant SimCtx as SimulationContext
    participant Sensor as BaseMultiMeshRayCaster
    participant Kernel as raycast_dynamic_mesh_slots_kernel

    Scene->>Cloner: make_clone_plan(sources, dests, num_envs, strategy)
    Cloner-->>Scene: ClonePlan(sources[], destinations[], clone_mask)
    Scene->>SimCtx: set_clone_plan(plan)
    Scene->>Scene: clone_environments() → usd_replicate / physics_clone_fn

    Sensor->>SimCtx: get_clone_plan()
    SimCtx-->>Sensor: ClonePlan
    Sensor->>Sensor: _initialize_warp_meshes_from_clone_plan(plan)
    Note over Sensor: Builds flat slot arrays

    loop each simulation step
        Sensor->>Sensor: _update_mesh_transforms()
        Sensor->>Kernel: "launch(dim=(num_slots, num_rays))"
        Note over Kernel: slot_id → tid_env via slot_env_ids
        Kernel-->>Sensor: ray_hits, ray_distance, ray_mesh_id
    end

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[greptile-apps]

Stale dest_template reference on refactored ClonePlan

_find_covering_plan accesses plan.dest_template, but the ClonePlan dataclass was restructured in this PR to replace the single dest_template: str field with destinations: list[str] (one entry per source row). Any caller of walk_target_prototypes will hit an AttributeError at runtime. The function needs to iterate over plan.destinations rows and use each row's template string to compute the prefix, rather than calling plan.dest_template.

[greptile-apps]

Test helper uses old ClonePlan API and old set_clone_plans method

This block calls sim.set_clone_plans(...) (plural), which was renamed to set_clone_plan(...) (singular) in this PR, and constructs ClonePlan(dest_template=env_fmt, prototype_paths=[...], ...) with field names that no longer exist (replaced by sources and destinations). Running this check script would immediately fail with TypeError at ClonePlan construction and AttributeError on set_clone_plans. It should use sim.set_clone_plan(ClonePlan(sources=[...], destinations=[...], clone_mask=...)).

[greptile-apps]

ray_mesh_id stores global slot index, not per-env mesh index

In raycast_dynamic_meshes_kernel (the rectangular-table variant), ray_mesh_id is set to wp.int16(tid_mesh_id) — the per-env mesh index. Here the flat-slot kernel stores wp.int16(slot_id) — the global slot index, which is different for environments that start at a non-zero slot offset. Any consumer that treats ray_mesh_ids as a dense per-env mesh index (e.g. to index into a per-env property table) will receive incorrect results for environments whose slots do not start at 0. The docstring for raycast_dynamic_mesh_slots_kernel should call out this semantic difference, and callers building on ray_mesh_ids should be verified against the new convention.