When will THC_GENERIC_FILE defined?

In file torch/csrc/generic/serialization.cpp,

#ifndef TH_GENERIC_FILE
#define TH_GENERIC_FILE "torch/csrc/generic/serialization.cpp"
#else

#ifdef THC_GENERIC_FILE
#include <c10/cuda/CUDAGuard.h>
#endif

I want to know when I use cuda, how THC_GENERIC_FILE defined?