Handle non-python builds

[dfalbel]

It seems that torchvision doesn't really support building the C++ library without linking to Python (see pytorch/vision#2692 ). To handle this we have a few options:

• Patch libtorchvision so it doesn't require linking to Python. I have tried this approach and that wasn't too complicated, only commenting a few #include <Python.h> and modifying the cmake build infrastructure to make Python optional. (see dfalbel/vision@2a81375)
• Vendor the parts of the code that interest us, currently mostly only the ops and possibly io and arrange our own build system for that. ops don't require Python at all, so we would be good with that.
• Ask PyTorch Vision maintainers to support non-python builds. They look interested in doing that given the comments in Using torchvision C++ API without installing python pytorch/vision#2692 but the issue is sitting since 2020 there.

[dfalbel]

Fixed with: pytorch/vision#5190