Pin downloads to release version

[glenn-jocher]

Fixes a release version to avoid forward-compatibility issues in future releases.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced model downloading with better URL checks and GitHub API integration.

📊 Key Changes

• 🔍 Added is_url function to verify if a URL points to an existent online file.
• 💾 Modified attempt_download function to:
  • Integrate with GitHub API to fetch available assets and tag names for specific release versions.
  • Handle specifying of release versions (e.g., 'v6.1') more flexibly.
  • Use a fallback list of assets and try to retrieve the latest git tag as a default if errored.

🎯 Purpose & Impact

• 🏗 Makes downloading models more robust by checking the existence of files before attempting to download.
• 🔄 Allows users to download specific versions of pretrained models easily, enhancing user experience and reproducibility.
• 🔄 Potential reduction in download-related issues, ensuring more stability in the setup process for new users.