Support sklearn models with multiple outputs

[abigailgold]

(i.e., nb_classes is an array instead of an integer).

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