Allow YOLOv5 execution from arbitrary cwd

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhancements to file path handling for improved relative path support in YOLOv5 scripts.

📊 Key Changes

• Default paths for weights, data, and project directories in detect.py, export.py, train.py, and val.py are now relative to the current working directory.
• Conversion of source to string format for compatibility in detect.py.
• Additional calls to check_yaml and check_file functions to validate YAML and file paths in train.py and val.py.

🎯 Purpose & Impact

• These changes aim to reduce path errors when scripts are executed from different directories, increasing the robustness and user-friendliness of the codebase.
• The updates standardize the way paths are handled across different scripts, making it easier to understand and maintain the code.
• Users should experience fewer issues with file not found errors, especially when running the code in various environments or directory structures. 🛠️📁