Update the docstrings to match the actual code defaults

[ytl0623]

Fixes #8689 .

Description

Update the docstrings to match the actual code defaults.

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]

Caution

Review failed

The pull request is closed.

📝 Walkthrough

Walkthrough

The focal loss module has been extended to support per-class alpha weighting. The FocalLoss class and softmax/sigmoid focal loss functions now accept alpha as either a scalar or a sequence of class weights, with updated signatures and validation logic. Scalar alpha follows the standard background/other-class weighting pattern, while sequence alpha applies per-class weights with appropriate broadcasting. In unified focal loss, docstrings were corrected to reflect actual gamma defaults: 2.0 for AsymmetricFocalLoss and 0.5 for AsymmetricUnifiedFocalLoss. Tests now verify alpha sequence broadcasting across multiple device and configuration combinations.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

✨ Finishing touches

• 📝 Generate docstrings

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📜 Recent review details

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Knowledge base: Disabled due to Reviews -> Disable Knowledge Base setting

📥 Commits

Reviewing files that changed from the base of the PR and between 57fdd59 and 9546f8e.

📒 Files selected for processing (3)

• monai/losses/focal_loss.py
• monai/losses/unified_focal_loss.py
• tests/losses/test_focal_loss.py

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