Add URL file download to check_file()

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced file check utility with download support for remote URLs.

📊 Key Changes

• 📁 Extended check_file function to handle remote URLs by downloading the file if it doesn't exist locally.
• 🛠️ Added an assertion to ensure the downloaded file exists and is non-empty, to prevent downstream errors.
• ✨ Improved error messages, clarifying when a file isn't found or when multiple files match the search pattern.

🎯 Purpose & Impact

• 🚀 Users can now reference files hosted remotely with ease, as they will be automatically downloaded when not present locally.
• ☑️ Robustness of file handling is improved, with checks to avoid incomplete downloads leading to smoother user experiences.
• 📝 Clearer error messages aid users in debugging path issues, reducing confusion and support requests.

[glenn-jocher]

/rebase

[glenn-jocher]

@kalenmike this PR now implements direct from URL commands for --data, --cfg, --hyp training arguments, i.e.:

python train.py --data https://url/to/data.yaml

I'll handle --weights later today.

[glenn-jocher]

Example usage / test result: