init: starting implementation of ignore_index #8734

[Rusheel86]

Fixes #8734

Description

This PR introduces native support for ignore_index in core losses and metrics, as requested. The goal is to allow voxels with specific labels (e.g., padding or unlabeled regions) to be excluded from calculations, which is critical for medical imaging workflows.

Current focus: Implementing masking logic in DiceLoss. Subsequent updates will cover DiceMetric, MeanIoU, and other requested components.

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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