Fix IndexError in adaptive threshold computation

[djdameln]

Description

• This PR addresses a bug causing validation to fail when computing the adaptive threshold. In rare cases (e.g. few anomalous images in validation set), it is possible that the recall is 1.0 for all threshold values included in TorchMetrics precision-recall curve computation. In this case, the thresholds tensor gets reduced to a scalar value, and trying to retrieve the first item produces an IndexError. In this case we can safely return the scalar threshold value instead.

Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• My code follows the pre-commit style and check guidelines of this project.
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes

[samet-akcay]

thanks for spotting this!