DNN: supports NonMaxSuppression operator from ONNX

[fengyuentau]

Fixes #19779. And one step forward to support the ssd-mobilenet from onnx model zoo.

opencv/opencv_extra#1005

TODO list:

• Supports the case where exporting a PyTorch model with torchvision.ops.nms to ONNX.
• Supports the case where exporting a PyTorch model with torchvision.ops.batched_nms to ONNX.

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[fengyuentau]

Since dnn does not support dynamic shape for now, the number of output boxes have to be set fixed (650 is used in this implementation, but there will not be so many boxes in one single image typically speaking). So invalid indices have to be set to -1.

@rogday if the nms operator is ported from torchvision.ops.nms, the NonMaxSuppression operator is always followed by a Gather operator (see the visualization in opencv/opencv_extra#1005). I wonder if it is reasonable to drop those invalid indices in your pull request for the Gather operator.

[fengyuentau]

I would like to postphone this pull request, since I cannot find a workaround for the dynamic output of nms. Will get back to this pull request when we have next gen dnn which supports dynamic shape.

[dkurt]

Can this op fit to DetectionOutput layer?

[fengyuentau]

Can this op fit to DetectionOutput layer?

Yes and no. They all do NMS but DetectionOutput layer tries to generate priors and decode bboxes as well, which are not needed in the NonMaxSuppression operator from ONNX. Another problem is DetectionOutput layer has topK and keepTopK to limit the number of output bboxes, but NonMaxSuppression does not. So the output is dynamic for NonMaxSuppression and therefore this PR has to be postponed util we have dynamic shape support in the next generation of DNN module.

[dkurt]

And one step forward to support the ssd-mobilenet from onnx model zoo.

I were not able to find NonMaxSuppression layers in SSD-MobilenetV1 and SSD-MobilenetV1-12 models.

[fengyuentau]

I were not able to find NonMaxSuppression layers in SSD-MobilenetV1 and SSD-MobilenetV1-12 models.

It is in the subgraph of the second Loop operator in SSD-MobilenetV1-12 for example.

[dkurt]

@fengyuentau, cannot find it, maybe model has been updated?

[fengyuentau]

Click on Loop and on the pop-up attribute page you can see there is a body attribute. Click on the name of the body attribute and you will be redirected to the subgraph, where you can find the NonMaxSuppression operators.