GitHub API rate limit fallback

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced flexibility in downloading model files from custom repositories.

📊 Key Changes

• 🔄 Updated the attempt_download function to allow for specifying a custom GitHub repository.
• 🛠 Added a fallback mechanism that defaults to a predefined asset list and uses the last git tag in case the GitHub API call fails.
• 📡 Included a dynamic repository URL for storage bucket downloads, aligning with the new custom repo feature.

🎯 Purpose & Impact

• 🚀 Purpose: This change allows users to download model files from different GitHub repositories, not just the default ultralytics/yolov5. It also provides a robust fallback for download attempts if the GitHub API request fails.
• ✨ Impact: Users and developers can work with models across various repositories, increasing modularity and collaboration opportunities. Moreover, the fallback ensures model downloads are less likely to fail, improving the reliability of model setup processes.