Getting started/building from source does not make it clear what options to use for CUDA

[nmm0]

When using the CUDA backend, the user either needs to:

• Set CMAKE_CXX_COMPILER to the path to nvcc wrapper
• Set CMAKE_CUDA_COMPILER to nvcc and enable Kokkos_ENABLE_COMPILE_AS_CMAKE_LANGUAGE

The instructions at https://kokkos.org/kokkos-core-wiki/get-started/building-from-source.html do not make this clear, and seem to imply with this sentence:

For example, use hipcc for AMD GPUs, icpx for Intel GPUs, or g++ or clang++ for CPUs.

That one should set CMAKE_CXX_COMPILER to nvcc, which is incorrect

[masterleinad]

When using the CUDA backend, the user either needs to:

• Set CMAKE_CXX_COMPILER to the path to nvcc wrapper
• Set CMAKE_CUDA_COMPILER to nvcc and enable Kokkos_ENABLE_COMPILE_AS_CMAKE_LANGUAGE

You can also just set CMAKE_CXX_COMPILER to whatever (host) compiler you want. We will implicitly use nvcc_wrapper if that compiler isn't nvcc_wrapper or clang++. Ultimately, only Cuda is special in this way.