ncnn/tools/pytorch

You can find pytorch2ncnn tool here

https://github.com/starimeL/PytorchConverter
https://github.com/inisis/brocolli

For newer version, pytorch already supports exporting to onnx out of box
You can follow the practical guide here

https://github.com/Tencent/ncnn/wiki/use-ncnn-with-pytorch-or-onnx