--resume training from URL weights fix

[glenn-jocher]

@kalenmike should fix data error on HUB resume

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improvement in data path handling for resumed YOLOv5 training sessions.

📊 Key Changes

• Fixed the reference to the opt_data variable, ensuring the correct data path is used.
• Replaced a simple string conversion with the check_file function for validating the data path.

🎯 Purpose & Impact

• 🛠 Fixes a bug where the data path might not be properly set when resuming training, enhancing reliability.
• ✅ Ensures that resumed training sessions start smoothly without authentication timeout issues, improving user experience for those training models with YOLOv5 over time.