Configure reference frame for multi-frame video clips

[djdameln]

Description

• In multi-frame video pipelines, we need to set which frame acts as the reference frame, meaning the ground truth information of that frame will be used to represent the (multi-frame) video clip.
• This PR makes this configurable from the config. The user may elect to take the GT info from the first frame, last frame, mid frame or all frames combined.

Changes

• Bug fix (non-breaking change which fixes an issue)
• Refactor (non-breaking change which refactors the code base)
• 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
• I have added a summary of my changes to the CHANGELOG (not for minor changes, docs and tests).

[samet-akcay]

Thanks, only few comments

[ashwinvaidya17]

Thanks. One minor comment

[ashwinvaidya17]

Thanks. Looks good to me