Add PyTorch-only benchmark arg

[glenn-jocher]

Usage:

python utils/benchmarks.py --weights yolov5s.pt --img 640 --pt-only

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Added option to benchmark PyTorch models exclusively.

📊 Key Changes

• New argument pt_only added to the benchmarking utility.
• The benchmarking process can be terminated early after testing PyTorch if pt_only is set.
• CLI now accepts --pt-only as an option to enable this new feature.

🎯 Purpose & Impact

• 🎯 Purpose: Simplifies benchmarking for users who only want to test the performance of PyTorch models without completing benchmarks for other export formats.
• ✅ Impact: Users can save time and resources during benchmarking when only PyTorch model performance is of interest, making the process more efficient and tailored to specific needs.