Dataset download bash script updates

[glenn-jocher]

This refactors the bash download scripts with improvements. VOC data is now also self-hosted on https://github.com/ultralytics/yolov5/releases/download/v1.0/, which should improve availability, as the official site and some mirrors were intermittently unavailable.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Optimizing dataset download scripts for COCO and VOC datasets.

📊 Key Changes

• Streamlined the COCO dataset download/unzip process by consolidating commands into fewer lines.
• Simplified the VOC dataset download script, removing unnecessary navigational steps and conditionals.
• Both scripts now leverage a loop to handle multiple file operations, improving readability and maintainability.
• Standardized URLs for dataset downloads to make them easily changeable and consistent.

🎯 Purpose & Impact

• Makes the dataset downloading process more efficient and user-friendly.
• Reduces potential issues with directory navigation and conditionals, leading to a smoother setup experience.
• Improves developers' and users' efficiency when working with large image datasets, which is crucial for machine learning tasks. 🚀