Adds USD-level randomization to event manager

[Mayankm96]

Description

Certain scene-level randomizations (such as randomizing the scale) must happen before the simulation starts playing. To this end, the MR adds a new event mode called "scene," which gets called right after the scene design is complete.

Since the event manager CANNOT be initialized before the simulation starts playing, the current implementation hacks the event mode into the environment constructor directly. This is not ideal but at this point, there is no other way around it.

Type of change

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

Screenshots

If you execute:

./isaaclab.sh -p source/standalone/tutorials/03_envs/create_cube_base_env.py --num_envs 32

Output:

Checklist

• 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 updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[Mayankm96]

I already spent a day trying to figure out multiple ways to initialize the event manager before the sim starts playing. The scene entity config resolution is impossible until physics parses the scene (since we need the PhysX body and joint ordering). It is rather unfortunate, but that's how it is right now.

The functionality still works, so if it makes sense, we can move forward with it.