Support sklearn models with multiple outputs

[abigailgold]

Description

The Classifier nb_classes setter now accepts non-integer values to support cases where the classifier has multiple outputs.
check_and_transform_label_format still only supports integer nb_classes values, but throws a TypeError if a different type is received.

Type of change

Please check all relevant options.

• Improvement (non-breaking)
• Bug fix (non-breaking)
• New feature (non-breaking)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Testing

Added a test with a multi-output DecisionTreeClassifier

Test Configuration:

OS: MacOS 14.4
Python version: 3.9
ART version or commit number
TensorFlow / Keras / PyTorch / MXNet version

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• My changes have been tested using both CPU and GPU devices

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.40%. Comparing base (0b4bb68) to head (bd58b1a).
Report is 7 commits behind head on dev_1.19.0.

Files with missing lines	Patch %	Lines
art/estimators/classification/classifier.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           dev_1.19.0    #2505      +/-   ##
==============================================
- Coverage       85.42%   85.40%   -0.02%     
==============================================
  Files             334      334              
  Lines           30998    31000       +2     
  Branches         5298     5299       +1     
==============================================
- Hits            26479    26476       -3     
- Misses           3052     3055       +3     
- Partials         1467     1469       +2     

Files with missing lines	Coverage Δ
art/utils.py	79.57% <100.00%> (+0.05%)	⬆️
art/estimators/classification/classifier.py	96.61% <0.00%> (ø)

... and 3 files with indirect coverage changes

[beat-buesser]

Hi @abigailgold Thank you very much for your contribution!