Update FP16 --half argument for test.py and detect.py

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improved command-line argument parsing for half-precision inference.

📊 Key Changes

• Changed the --half argument to use action='store_true' instead of type=bool for better command-line usability.

🎯 Purpose & Impact

• Enhances Usability: Simplifies how users enable half-precision inference by just including the flag (--half) rather than specifying a boolean value.
• Avoids Confusion: Prevents potential user errors where the incorrect passage of boolean values could lead to unexpected behavior.
• Impact on Users: Users can more intuitively activate FP16 inference to potentially speed up model inference on compatible hardware.