--resume training from URL weights

[glenn-jocher]

@kalenmike HUB resume fix PR

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Refinement of the resume functionality in YOLOv5 training script.

📊 Key Changes

• Simplified the code for resume option in training to make it easier to understand and maintain.
• Replaced a manual check of the checkpoint file existence with a call to check_file() function.

🎯 Purpose & Impact

• 💪 Purpose: The update aims to make resuming training more reliable and user-friendly, minimizing errors caused by incorrectly specified resume files.
• 🔍 Impact: Users should experience smoother resuming of training processes, ideally with fewer errors or interruptions due to checkpoint file issues. This can save time and resources, especially in longer training runs.