Solved by Xander_Dunn in post #2 
              
              
                Lord have mercy. It was this in my CMakeLists.txt: 
set(CMAKE_CXX_FLAGS "-fsanitize=undefined -fsanitize=address")

leftover from some debugging. 
Sanitizers breaking CUDA isn’t too surprising.  
libtorch runs fine on GPU in the absence of these compiler flags.