[Feature]: Dynamic morph variant loading for heterogeneous entity

[Milotrince]

What feature or enhancement are you proposing?

Support loading different morphs for a heterogeneous entity after the scene is built.

Currently:

Variant Assignment Rules:
    When passing a list of morphs to scene.add_entity(), variants are distributed
    across environments using the following rules:

    1. When n_envs >= n_variants:
       Balanced block assignment. Environments are divided into blocks, with each
       block assigned to one variant. For example, with 4 variants and 8 environments:
       - Environments 0-1 -> Variant 0
       - Environments 2-3 -> Variant 1
       - Environments 4-5 -> Variant 2
       - Environments 6-7 -> Variant 3

    2. When n_envs < n_variants:
       Each environment i gets variant i (0-indexed). Variants beyond n_envs are
       unused. For example, with 4 variants and 2 environments:
       - Environment 0 -> Variant 0 (first morph in list)
       - Environment 1 -> Variant 1 (second morph in list)
       - Variants 2 and 3 are unused

Desiderata:

The same as above, and
    2. When n_envs < n_variants:
       All variants are preloaded. *Initially*, variants beyond n_envs are unused.
       Calling a function entity.load_morph_variants(variants_idx, envs_idx) makes it active.

Motivation

Want to be able to sample from many different objects (N) while training with num_envs < N. Currently, heterogeneous entity assigns a different obj per environment on build, so cannot load more kinds of objects than there are num_envs

Potential Benefit

Allow for training with larger datasets of objects, like https://arxiv.org/pdf/2504.05287 (which uses RaiSim which is cpu based)

What is the expected outcome of the implementation work?

• A function on entity, e.g. entity.load_morph_variants(variants_idx, envs_idx), which allows for changing the active morph.
• If needed, a new options on entity, enable_variant_loading
• Preprocessed geoms are stored as file in .cache

Additional information

Isaac sim multi asset spawning: https://isaac-sim.github.io/IsaacLab/main/source/how-to/multi_asset_spawning.html -- see Rigid Object Collections