Hash semantic ids to improve distinguishability of semantic segmentation images

[huidongc]

Description

Hash each semantic ID so nearby IDs (e.g. 1, 2, 3) map to visually distinct colors in the output.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Screenshots

Before the fix, the semantic segmentation images are fully transparent and nearly black.

Before	After

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