cmake : fix CMake requirement for CUDA

[cebtenzzre]

CMAKE_CUDA_ARCHITECTURES wasn't added until CMake 3.18.

Setting the minimum version of CMake to 3.18 also sets CMP0104 to NEW, which causes CMAKE_CUDA_ARCHITECTURES to be automatically set by CMakeDetermineCUDACompiler. So for the NOT DEFINED check to work, we must set the default before enable_language(CUDA).

[slaren]

I tried bumping the cmake_minimum_required to 3.18 (without other changes) and the value of CMAKE_CUDA_ARCHITECTURES is still respected.

[cebtenzzre]

the value of CMAKE_CUDA_ARCHITECTURES is still respected.

The default is not. If you change cmake_minimum_required(VERSION 3.17) in the CUDA branch to VERSION 3.18 (this line takes priority), grep -m1 nvcc build/compile_commands.json shows only compute_52,sm_52 on my system instead of compute_52,sm_52, compute_61,sm_61, and compute_70,sm_70 which is what should be there.