Add background class for segmentation if pixels without annotations found

[sokovninn]

This pull request introduces logic to handle unassigned pixels in segmentation masks.

Tested with a luxonis-train segmentation model training.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.12%. Comparing base (6bcbb11) to head (69e23f0).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #191      +/-   ##
==========================================
- Coverage   92.33%   92.12%   -0.22%     
==========================================
  Files          79       79              
  Lines        4161     4176      +15     
==========================================
+ Hits         3842     3847       +5     
- Misses        319      329      +10     

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[klemen1999]

LGTM