fix: add background to multiclass only

[JSabadin]

Fix: Background Class for Multiclass Segmentation Only

Overview

In this update, we improved how background classes are handled in segmentation tasks within the LuxonisLoader.

• Binary Segmentation: The background class is not added.
• Multiclass Segmentation: The background class is added only when needed (multiclass segmentation)

Performance and Testing

We validated this change with the following tests:

• Binary Segmentation: Training runs without errors or metric inconsistencies.
• Overfitting Test: Conducted on a cows segmentation dataset of 200 images for 10 epochs:
  • From scratch: Achieved a Jaccard index of 0.82.
  • Using pretrained weights: Achieved a Jaccard index of 0.98.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.47%. Comparing base (6bcbb11) to head (dd106d7).
Report is 12 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #204      +/-   ##
==========================================
+ Coverage   92.33%   92.47%   +0.14%     
==========================================
  Files          79       80       +1     
  Lines        4161     4227      +66     
==========================================
+ Hits         3842     3909      +67     
+ Misses        319      318       -1     

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