Fix: make y optional in ignore_background

[Kirscher]

Fixes # .

Description

This PR addresses a TODO in monai/metrics/active_learning_metrics.py by making the y argument optional in the ignore_background utility function. This allows for cleaner code when only y_pred needs to be processed, as seen in active_learning_metrics.py.

Types of changes

• [x] Non-breaking change (fix or new feature that would not break existing functionality).
• [ ] Breaking change (fix or new feature that would cause existing functionality to change).
• [x] New tests added to cover the changes.
• [ ] Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• [x] Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• [ ] In-line docstrings updated.
• [ ] Documentation updated, tested make html command in the docs/ folder.

[coderabbitai[bot]]

Walkthrough

The ignore_background function in monai/metrics/utils.py now accepts an optional y parameter (defaulting to None) with updated return type tuple[NdarrayTensor, NdarrayTensor | None]. The calling code in monai/metrics/active_learning_metrics.py passes y=None to ignore_background when include_background is False, discarding the second return value.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

• Focused change across 2 files
• Straightforward parameter signature update with None guard
• Consistent calling pattern change
• No complex logic modifications

Pre-merge checks and finishing touches

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	Title clearly and concisely describes the main change: making the y parameter optional in ignore_background function.
Description check	✅ Passed	Description covers the core change, rationale, and test coverage. Author marked test addition and quick tests passing, though integration tests and docstring updates are unchecked.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

✨ Finishing touches

• [ ] 📝 Generate docstrings

🧪 Generate unit tests (beta)

• [ ] Create PR with unit tests
• [ ] Post copyable unit tests in a comment

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