Add average calibration losses

[theo-barfoot]

Description

Adds differentiable average calibration error losses for segmentation:

• HardL1ACELoss, using hard confidence bins
• SoftL1ACELoss, using differentiable soft bin assignments

The implementation follows the publication formulation, supports binary and multiclass spatial predictions, matches MONAI calibration-metric aggregation semantics, and handles empty classes consistently with the reference implementation. The losses are exported through monai.losses and documented in the losses reference.

This is part of #8505 and builds on the calibration metrics and handler introduced in #8707.

Validation

• python -m pytest -q tests/losses/test_calibration_loss.py tests/metrics/test_calibration_metric.py tests/handlers/test_handler_calibration_error.py
  • 47 passed, 1 skipped, 32 subtests passed
• ./runtests.sh --black --isort --ruff --pyrefly --copyright
• TorchScript, CPU/CUDA dtype, autocast, MetaTensor, non-contiguous input, compilation, output, and gradient parity checks

A companion 3D segmentation tutorial PR will demonstrate the losses on Medical Segmentation Decathlon Task04 Hippocampus.

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📥 Commits

Reviewing files that changed from the base of the PR and between 5384817 and 8526ca7.

📒 Files selected for processing (2)

• monai/losses/calibration.py
• tests/losses/test_calibration_loss.py

🚧 Files skipped from review as they are similar to previous changes (2)

• monai/losses/calibration.py
• tests/losses/test_calibration_loss.py

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📝 Walkthrough

Walkthrough

This change adds HardL1ACELoss and SoftL1ACELoss with hard and interpolated binning, input validation, weighting, reductions, activation support, and empty-class handling. It exports both losses, adds documentation and comprehensive tests, and updates calibration citations to the 2026 IEEE publication.

Estimated code review effort: 4 (Complex) | ~60 minutes

Merge Risk: ⚪ Minimal · up to 8526c

This change adds public calibration-loss implementations with accompanying tests and documentation. No concrete merge-blocking risk is currently identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 15.15% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 33 functions across 5 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: adding average calibration losses.
Description check	✅ Passed	The description clearly explains the new losses, supported behavior, exports, documentation, and validation results. It omits the template's "Fixes #" line and "Types of changes" checklist, but the co…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Description check

Explanation

The description clearly explains the new losses, supported behavior, exports, documentation, and validation results. It omits the template's "Fixes #" line and "Types of changes" checklist, but the core information is complete.

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[theo-barfoot]

Companion 3D segmentation calibration tutorial: Project-MONAI/tutorials#2072

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Actionable comments posted: 1

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Inline comments:
In `@monai/losses/calibration.py`:
- Around line 112-113: Update the weight validation in the loss constructor,
including both class_weight and empty_weight checks, to reject any non-finite
values such as NaN and infinity in addition to negative values. Preserve the
existing ValueError behavior for invalid weighting parameters, and add
constructor tests covering NaN and infinity for each supported weight input.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

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📥 Commits

Reviewing files that changed from the base of the PR and between 9ea04d4 and 5384817.

📒 Files selected for processing (6)

• docs/source/losses.rst
• monai/handlers/calibration.py
• monai/losses/__init__.py
• monai/losses/calibration.py
• monai/metrics/calibration.py
• tests/losses/test_calibration_loss.py

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