Add --keras argument for TF exports

[glenn-jocher]

Resolves #7911 (comment)

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced export capabilities with Keras model support in YOLOv5.

📊 Key Changes

• Added a keras flag to enable Keras model export functionality.
• Incorporated Keras export logic into the model export process.
• Updated command-line argument parser to accept the new --keras parameter for triggering Keras exports.

🎯 Purpose & Impact

• Purpose: This change allows users to export YOLOv5 models as Keras models, providing additional flexibility.
• Impact: Users can now integrate YOLOv5 with applications and systems that operate better with Keras models, potentially widening the framework's adoption and making it more accessible in different machine learning ecosystems. 🚀