Fix semantic seg ground truth mask extraction

[vinnamkim]

Summary

• Let ignore_index working correctly.
• Merge individual masks into a class mask (semantic mask) correctly.

How to test

Added an unit test to validate ignore_index

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added e2e tests for validation.
• I have added the description of my changes into CHANGELOG in my target branch (e.g., CHANGELOG in develop).​
• I have updated the documentation in my target branch accordingly (e.g., documentation in develop).
• I have linked related issues.

License

• I submit my code changes under the same Apache License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below).

# Copyright (C) 2024 Intel Corporation
# SPDX-License-Identifier: Apache-2.0

[codecov]

Codecov Report

Attention: Patch coverage is 89.18919% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 79.71%. Comparing base (0db2e80) to head (023d3ee).

Files	Patch %	Lines
src/otx/core/data/dataset/segmentation.py	88.57%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3270      +/-   ##
===========================================
+ Coverage    79.69%   79.71%   +0.02%     
===========================================
  Files          180      180              
  Lines        15365    15398      +33     
===========================================
+ Hits         12245    12275      +30     
- Misses        3120     3123       +3     

Flag	Coverage Δ
py310	79.71% <89.18%> (+0.02%)	⬆️
py311	79.71% <89.18%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.