Added method for spawning multi-asset

[Kyu3224]

Description

Regarding issue #3070, I added support for spawning objects in each environment within a multi-USD (or asset) setup using various methods. Users can define custom functions in wrappers/utils.py or any other desired location, specify them in the config, and have them dynamically loaded using hydra.utils.get_method. The loaded choice_fn will then iterate through all prim_paths and assign asset indices based on the defined rules.

Following @Mayankm96 's advice, this design allows for user-defined logic to determine how assets are distributed across environments, providing both built-in methods (e.g., sequential, split) and the flexibility to extend with custom strategies.

I aimed to align this as closely as possible with the Isaac Lab code style and direction, but I'm not sure if it worked well, so I would appreciate your feedback. Basically, I have implemented sequential and split methods, so users can select between these two by default, and they can also add and use additional methods as needed.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• [X] I have run the pre-commit checks with ./isaaclab.sh --format
• [X] I have made corresponding changes to the documentation
• [X] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• [X] I have added my name to the CONTRIBUTORS.md or my name already exists there

[Kyu3224]

Hi @ooctipus , I think the suggestion about simplifying with random as an option is a great idea.

Regarding the idea about spawning rigidBodyCollection within multi-asset, could you give an example of how that would work? Currently, I’m focusing only on spawning articulation systems using different robots, but I think it could be extended to a more generalized approach.

[Kyu3224]

I tried to incorporate all the suggested comments as much as possible. Please let me know if there are any further improvements that can be made. Additionally, I added a section in the initialization to show the distribution of agents for each asset (30 environments with 4 assets).

[ooctipus]

Thank you @Kyu3224 this is very nice of you, and thank you @Mayankm96 for the nice suggestion!

[ooctipus]

@Kyu3224

Big Big sorry!!!!!

We are planning to do a big refactor on spawner + cloner that would 1. drop the requirement for using isaac_sim 2. allow more flexible combination of assets. 3. faster cloning time.

which means this PR will probably not be applicable soon.

Sorry again for all the work, I really appreciate reviewing your PR and discussions.

Feel free to close it, or I can help close it when the refactor comes in : )))