Example of how to use GenerateProposalsOp in C++ API?

I’m trying to get a Detectron mask-rcnn model running in C++ using the caffe2 API. There is almost no examples of how to use the caffe2 C++ API, but I followed this tutorials to be able to load these detectron models from the model zoo.

I think I need to use GenerateProposalsOp connected to some operator in the loaded nets, but I’m not sure, and I can’t find any example of how to use GenerateProposalsOp anywhere. Could anybody give me help/directions?