Generalize AsymmetricUnifiedFocalLoss for multi-class and align interface

[ytl0623]

Fixes #8603 .

Description

Addresses the limitations of AsymmetricUnifiedFocalLoss and its dependencies (AsymmetricFocalLoss, AsymmetricFocalTverskyLoss). The previous implementations were hard-coded for binary segmentation and lacked the standard sigmoid/softmax/to_onehot_y interface common to other MONAI losses.

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.