I forgot to mention, that I used the CMake package in site-packages/torch/share/cmake to specify the PyTorch dependency:
target_link_libraries(torch_plus_ops ${TORCH_LIBRARIES}).
_C.lib does not define the symbol either. Also, when using __declspec(dllexport), the linker would not look for the symbol in imported libraries, right?
I checked and succeeded building without the “Torch” CMake package and instead used the compiler/linker flags from torch.utils.cpp_extension.