Integrate u2net into model zoo about opencv_zoo

[fengyuentau]

Okay, I will try to add it in the following week.

[fengyuentau]

@jsxyhelu can u provide a link to the paper or github repo of u2net?

[fengyuentau]

For the human-segmentation u2net, I have create scripts to port u2net to ONNX (u2net_human_seg-2onnx.py) and perform inference using OpenCV DNN (u2net_huam_seg-onnx-opencv.py). Get these scripts: https://github.com/fengyuentau/U-2-Net/tree/onnx_export.

However, the model itself is too large (168MB) to put into our zoo. The size of models in this zoo should be lower than 100MB.

[jsxyhelu]

that is great,thanks!