[Fixes #9116] Include per-class rank in EmbeddingCollapseMetric aggregate score

[DucAnhValentinoNguyen]

Fixes #9116

Description

compute_embedding_collapse builds aggregate from a fixed primary key set that does not contain any per_class_rank_<cls> key, so per-class rank — the only indicator that can see collapse confined to one class — is computed, returned, and then excluded from every reduction.

Under reduction="max", documented as the worst-case score for safety-critical use, a class can be fully collapsed (per_class_rank_<cls> == 1.0) while aggregate reports a healthy embedding space, because the global indicators are all genuinely healthy.

This folds the worst per-class score into the reduction pool as a single term. Under max the result is the worst case across every indicator, including per-class. Under mean, reducing to one term first keeps one vote per indicator, so the average does not drift as the number of classes grows.

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.

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• tests/metrics/test_embedding_collapse.py

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• tests/metrics/test_embedding_collapse.py

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📝 Walkthrough

Walkthrough

compute_embedding_collapse now adds the maximum non-null per-class rank score to the aggregate pool. The mean reduction counts this maximum as one term, regardless of class count. Documentation and tests describe and check these behaviors.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to 9d06f

The aggregate now reflects the worst per-class rank without weighting the mean by class count. No issue identified here prevents merging after normal checks.

🚥 Pre-merge checks | ✅ 7

✅ Passed checks (7 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: including the per-class rank in the EmbeddingCollapseMetric aggregate score.
Description check	✅ Passed	The description includes the issue reference, change summary, change types, test coverage, quick-test status, and docstring update status.
Linked Issues check	✅ Passed	The pull request references issue #9116, and the implementation directly addresses the issue by including the worst per-class rank in aggregate reductions.
Out of Scope Changes check	✅ Passed	The changes are limited to EmbeddingCollapseMetric aggregation behavior, related docstrings, and focused tests. They match the stated objectives.
Linked Issues check	✅ Passed	Issue #9116 requires per-class rank to enter aggregate and remain one term under reduction="mean". The PR stores per-class scores separately, reduces them to the worst class, and adds that value t…
Out of Scope Changes check	✅ Passed	The changes address issue #9116 through aggregation logic, related docstrings, and automated tests. No unrelated changes are identified.
Docstring Coverage	✅ Passed	Docstring coverage is 85.71% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 2 files.

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Actionable comments posted: 1

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Inline comments:
In `@tests/metrics/test_embedding_collapse.py`:
- Around line 449-450: Add Google-style docstrings to
_majority_healthy_minority_collapsed with Args and Returns sections, and add
concise descriptive docstrings to
test_collapsed_minority_class_reaches_aggregate and
test_max_aggregate_at_least_worst_per_class. Leave the existing docstrings on
the other test methods unchanged.

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📒 Files selected for processing (2)

• monai/metrics/embedding_collapse.py
• tests/metrics/test_embedding_collapse.py

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[DucAnhValentinoNguyen]

Hi @ericspod @KumoLiu @Nic-Ma — checking in on this, it's been about two weeks. I've addressed CodeRabbit's docstring suggestion in the latest commit. I also notice the test workflows haven't run yet (only the DCO check has) — let me know if they need approval on your end, or if you'd like any changes. Thanks for your time!