Resume fix for yaml constructor posixpath error

[glenn-jocher]

This PR attempts to fix a --resume bug following PR #1377.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Streamlined file and directory handling in YOLOv5 training script.

📊 Key Changes

• 🛠 Changed opt.save_dir to always be a Path object for consistent directory handling.
• 🗑️ Removed automatic backup of weights during training to avoid unnecessary file duplication.
• 🚀 Adjusted checkpoint loading logic to directly use checkpoint directory path when resuming training.
• 📁 Standardized the use of Path objects when constructing file paths for improved clarity and maintainability.

🎯 Purpose & Impact

• 💾 These changes aim to simplify the codebase, reducing the potential for errors related to file paths.
• 🧹 By avoiding unnecessary weight backups, the training process is decluttered, saving disk space.
• 🛤️ Provides a clearer and more robust framework for file path operations, which should help users and developers avoid path-related bugs.
• 🚀 Enhancing maintainability and readability of the code can facilitate further development and user engagement with the YOLOv5 project.

[glenn-jocher]

--resume tests passing in Colab, merging.