Adds Isaac-Stack-Cube-Franka-IK-Rel-v0 task variants with varying stack orders

[nv-caelan]

Description

This PR adds 4 new Isaac-Stack-Cube-Franka-IK-Rel-v0 task variants corresponding to stacking [Red, Green], [Red, Green, Blue], [Blue, Green], and [Blue, Green, Red]. The objective is to create a test bed for different success conditions in the same scene.

Type of change

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

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 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

[greptile-apps]

Greptile Summary

This PR adds 4 new Isaac Lab manipulation task variants for stacking cubes in different color orders (Red-Green, Red-Green-Blue, Blue-Green, Blue-Green-Red), creating a testbed for evaluating different success conditions in the same scene. The cubes_stacked termination function is cleanly refactored to make the third cube optional (cube_3_cfg: SceneEntityCfg | None), enabling 2-cube stacking checks while remaining fully backward-compatible with the existing 3-cube default. The cube-to-color mappings (cube_1=Blue, cube_2=Red, cube_3=Green) are correctly applied across all new configs, and the new environment registrations are properly configured.

Key changes:

• terminations.py: cube_3_cfg parameter made optional with an if cube_3_cfg is not None guard; type annotations added to atol, rtol, and return type
• stack_ik_rel_env_cfg.py: 4 new @configclass environments each override terminations.success with the correct cube order
• __init__.py: 4 new gym.register entries added with correct entry points
• All changes are logically correct and the refactoring is backward-compatible

Confidence Score: 5/5

• This PR is safe to merge with no identified issues. All new task variants are logically correct with proper cube-color mappings and the termination logic refactoring is backward-compatible.
• The core implementation is verified correct: (1) the cubes_stacked refactor properly makes the third cube optional with correct conditional logic, (2) all 4 new environment configs correctly map cube entities to their intended stacking order, (3) gym registrations are properly configured with correct entry points, and (4) existing functionality remains intact. No logic errors, syntax issues, or safety concerns identified.
• No files require special attention

_{Last reviewed commit: a323142}

[njawale42]

Looks good to me!