Remove opt from create_dataloader()`

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhancing support for single-class datasets in YOLOv5 training and testing.

📊 Key Changes

• single_cls option now consistently passed directly as a parameter to various functions instead of being extracted from opt.
• The create_dataloader function signature is updated to receive single_cls as a parameter.

🎯 Purpose & Impact

• The refactor improves code clarity and maintainability by using the single_cls parameter explicitly rather than indirectly through opt.
• This change should not affect multi-class dataset handling but ensures single-class datasets are handled correctly across different parts of YOLOv5, providing more robust support for users working with such datasets.