Generalized regression renaming

[glenn-jocher]

This PR substitutes 'GIoU' and 'giou' with 'box' throughout YOLOv5. This change is required as we currently use CIoU for box regression criterion rather than GIoU, and using the general term 'box' allows for future changes in the regression criterion without requiring subsequent renaming.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Update loss terminology from GIoU to box for clarity in codebase.

📊 Key Changes

• Renamed giou loss to box loss across various configuration and code files.
• Updated comments and logging to reflect the change from GIoU to box.
• Loss calculation logic now refers to iou loss instead of giou.

🎯 Purpose & Impact

• Clarifies Loss terminology: Simplifies understanding by using a more generic term box which encompasses different IoU-based loss methods (like CIoU which is actually used).
• Consistency: Maintains consistency in how loss is referred to within the code, potentially reducing confusion.
• No Functional Change: The update is predominantly cosmetic, as it does not alter the underlying logic; the functional impact on users is minimal.

[glenn-jocher]

CI tests are passing, will merge.

[glenn-jocher]

BREAKING CHANGE WARNING

Users of earlier/custom hyperparameter files must rename 'giou' field to 'box'.