Rename onnx_dynamic -> dynamic

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improvement in grid reconstruction logic for YOLOv5 model export.

📊 Key Changes

• Renamed the onnx_dynamic attribute to dynamic in the Detect class.
• Updated usage of the renamed attribute in the export.py script.

🎯 Purpose & Impact

• ✨ Clarification: The renaming makes the code clearer as dynamic is more general and not limited to just ONNX export scenarios.
• 🚀 Enhanced Functionality: This change potentially leads to better handling of grid reconstruction when exporting the model for different platforms, not just limited to ONNX.
• 📈 User Experience: Users benefiting from more intuitive attribute naming, along with potentially more reliable export functionality across different model formats.