Fix segmentation Dice + GeneralizedDice for 2d index tensors

[SkafteNicki]

What does this PR do?

Fixes #2816

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• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--2832.org.readthedocs.build/en/2832/

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 35%. Comparing base (bb1af09) to head (6f487cb).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2832     +/-   ##
========================================
- Coverage      69%     35%    -34%     
========================================
  Files         330     344     +14     
  Lines       18793   18969    +176     
========================================
- Hits        12940    6688   -6252     
- Misses       5853   12281   +6428