Add suffix checks

[glenn-jocher]

Add additional suffix checks on YAML files and weights files in train/val/detect. Clearer error messages should help remove ambiguity about what weights formats are accepted by which files.

Initiated due to #4708 (comment)

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced weight file and YAML configuration validation in YOLOv5 to improve reliability and user feedback.

📊 Key Changes

• Added function check_suffix() to validate weight file extensions.
• Added function check_yaml() to validate YAML files and ensure they have the correct .yaml or .yml suffix.
• Replaced check_file() calls with check_suffix() and check_yaml() where appropriate across various files (e.g., detect.py, train.py, and val.py).
• Updated import statements removing check_file and adding new utility functions.

🎯 Purpose & Impact

• Purpose: To ensure that model weights and configuration files provided by users are valid, preventing confusions or errors caused by incorrect file types.
• Impact: Users will experience more robust error handling and clear messaging when file extensions do not meet the expected criteria, leading to smoother setup and training processes in YOLOv5.