Make y optional in ignore_background function

[ytl0623]

Description

This PR addresses a TODO by refactoring the ignore_background utility function.

Types of changes

• 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).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• 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]

📝 Walkthrough

Walkthrough

ignore_background in monai/metrics/utils.py now accepts an optional y: NdarrayTensor | None = None and has typing overloads for both call patterns. It computes a background-removed y_pred_out (only when channel count > 1) and returns y_pred_out when y is None or (y_pred_out, y_out) when y is provided. The function docstring and typing.overload import were added. In monai/metrics/active_learning_metrics.py, compute_variance calls ignore_background with include_background=False and assigns the single returned tensor to y_pred (no longer expecting a tuple or reassigning y).

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	Title clearly and concisely summarizes the main change: making the y parameter optional in ignore_background.
Description check	✅ Passed	Description covers the main change and includes required template sections, though docstring and documentation update checkboxes remain unchecked.

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✨ Finishing touches

• 📝 Generate docstrings

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@monai/metrics/utils.py`:
- Around line 62-80: Update the ignore_background docstring to include a
Google-style Returns section describing the returned tuple (y_pred with
background channel removed when applicable, and y which may be None or have its
background removed) and the shapes/types returned; then add a unit test for the
function ignore_background that calls it with y=None (and representative y_pred
shapes for classification and segmentation) asserting it returns the expected
y_pred (background removed only when channels>1) and that y is still None,
ensuring no exceptions occur.

[ericspod]

Hi @ytl0623 thanks for this fix, I think it's fine as a minor change with a couple of comments.

[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@monai/metrics/utils.py`:
- Around line 80-91: Add unit tests covering both branches of the function that
uses y_pred, y, y_pred_out and y_out: (1) when y is None assert the function
returns a single tensor equal to y_pred (and when y_pred.shape[1] > 1 that it
returns y_pred[:,1:]); (2) when y is provided assert it returns a tuple
(y_pred_out, y_out) and that both are correctly sliced to [:,1:] when their
shape[1] > 1 and unchanged when shape[1]==1. Place tests to call the same
function containing this snippet, use small synthetic NdarrayTensor inputs to
validate shapes and values, and include assertions for returned types (single
tensor vs tuple).