Bugfix for GeneralizedDiceScore to yield NaN for missing classes

[VijayVignesh1]

What does this PR do?

Fixes GeneralizedDiceScore to yield NaN for missing classes when per_class=True.
Added testcases to verify the same.

Fixes #2846

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📚 Documentation preview 📚: https://torchmetrics--3251.org.readthedocs.build/en/3251/

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Codecov Report

❌ Patch coverage is 35.71429% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 32%. Comparing base (e08e009) to head (ef5dade).
⚠️ Report is 1 commits behind head on master.

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HEAD has 478 uploads less than BASE

Flag	BASE (e08e009)	HEAD (ef5dade)
torch2.0.1+cpu	24	3
python3.10	96	12
Windows	16	2
cpu	136	17
macOS	24	3
torch2.0.1	16	2
python3.12	32	4
torch2.8.0+cpu	24	3
Linux	96	12
torch2.7.1+cpu	16	2
torch2.8.0	8	1
torch2.2.2+cpu	8	1
torch2.3.1+cpu	8	1
torch2.4.1+cpu	8	1
torch2.1.2+cpu	8	1
torch2.6.0+cpu	8	1
torch2.5.1+cpu	8	1
python3.9	8	1
gpu	1	0
unittest	1	0

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #3251     +/-   ##
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- Coverage      69%     32%    -37%     
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  Files         364     349     -15     
  Lines       20096   19905    -191     
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- Hits        13790    6377   -7413     
- Misses       6306   13528   +7222     

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Let's revisit the range

[VijayVignesh1]

Done. Aggregating the numerator and denominator in the update function and finally dividing them in the compute function brings the range back to [0,1].

[Copilot]

Pull Request Overview

This PR fixes a bug in GeneralizedDiceScore where missing classes were not properly handled when per_class=True, causing incorrect behavior instead of returning NaN for those classes.

• Refactored the metric's state tracking to properly identify missing classes and return NaN for them
• Updated the compute logic to handle both per-class and aggregate scoring correctly
• Added comprehensive test coverage for missing class scenarios

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/torchmetrics/segmentation/generalized_dice.py	Refactored state management and compute logic to track class presence and return NaN for missing classes
src/torchmetrics/functional/segmentation/generalized_dice.py	Added NaN handling for non-per-class computation when denominator is zero
tests/unittests/segmentation/test_generalized_dice_score.py	Added test cases for missing classes and zero denominator scenarios
CHANGELOG.md	Updated changelog to document the bug fix

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[Borda]

@VijayVignesh1 dont worry, Copilot is not always right but it may spot some overlooked issues

[justusschock]

the tensorshape shouldn't change here!

[VijayVignesh1]

I've made the changes.

[justusschock]

also here the shape should not change.

[VijayVignesh1]

Done