Add OpenCV DNN option for ONNX inference

[glenn-jocher]

Usage:

# Export
python export.py --weights yolov5s.pt --include onnx --simplify

# Inference
python detect.py --weights yolov5s.onnx  # ONNX Runtime inference
python detect.py --weights yolov5s.onnx -dnn  # OpenCV DNN inference

NOTE: Requires cv2>=4.5.4

OpenCV(4.5.3)

Need to use latest version i.e. 4.5.4 which was released just a few days ago. I have added a fix for this exact error in opencv opencv/opencv#20713 which will be present in the 4.5.4 version.

opencv-python is still on 4.5.3 so we need to wait till the latest version is released which contains the fix. Once it is released,
pip install -U opencv-python should solve the issue.

Meanwhile, the following onnx models should also work with OpenCV 4.5.3:

python export.py --weights yolov5s.pt --include onnx --opset 11
or
python export.py --weights yolov5s.pt --include onnx --simplify

Originally posted by @SamFC10 in #4811 (comment)

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Added OpenCV DNN support for ONNX models in YOLOv5 detection.

📊 Key Changes

• Introduced a new parameter dnn to enable OpenCV's DNN module for ONNX model inference.
• Modified weights parameter conversion to string to ensure compatibility.
• Adjusted ONNX model loading to use OpenCV DNN when dnn flag is set.
• Updated ONNX inference path to perform a forward pass with OpenCV DNN if chosen.
• Added the dnn flag to the command-line argument parser.

🎯 Purpose & Impact

• Purpose:
  • To provide an alternative inference backend using OpenCV's DNN module for users with ONNX models.
  • To improve compatibility and flexibility in model deployment scenarios.
• Impact:
  • Users can now leverage the OpenCV DNN module for potentially faster inference on CPU or compatibility with OpenCV processing pipelines.
  • Offers additional deployment options that could be beneficial in various environments without relying solely on ONNX Runtime.

[msly]

python detect.py --weights yolov5s.onnx --dnn # OpenCV DNN inference