refactor: comm space generic datatype conversion

[dssgabriel]

This PR is based on #188 (so we'll need to merge that first) and leverages the newly introduced member type aliases on the comm space concept to implement a CommunicationSpace-generic datatype conversion API. The present PR supersedes #186.

The new implementation is generic over both the communication space and the base type to convert, and remains fully compile-time known.

Some notable changes:

• declarative-style: no cascading if constexpr statements (similar implementation strategy to e.g., std::is_integral)
• exposed as top-level function in the KokkosComm:: namespace
  • not an implementation detail anymore (the OSU perf tests were relying on it)
• added a datatype_for(T&&) utility so we can retrieve the datatype from a variable, e.g.:

  using CS = KokkosComm::DefaultCommunicationSpace;
  
  const int& ref = 42;
  CS::datatype_type type = KokkosComm::datatype_for<CS>(ref);

I have kept the mpi/impl/types.hpp file for now, as it defines the logic for creating MPI datatypes (in mpi_view_type), although it appears this functionality is currently unused in Kokkos Comm.

The remaining changes propagate this new API throughout the rest of the code base.

[dssgabriel]

I am unsure why the Open MPI builds fail in the CI. Everything builds and passes on my machine with Open MPI 5.0.8

Here is my workflow for anyone willing to try and reproduce:

spack load [email protected] [email protected] [email protected] [email protected]

mkdir -p build/_deps

# Kokkos 5.0.0 w/ OpenMP backend
git clone https://github.com/kokkos/kokkos.git --depth 1 --branch 5.0.0 build/_deps/kokkos-src
cmake -S build/_deps/kokkos-src -B build/_deps/kokkos-build-omp -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_EXTENSIONS=OFF -DKokkos_ENABLE_OPENMP=ON
cmake --build build/_deps/kokkos-build-omp -j
cmake --install build/_deps/kokkos-build-omp --prefix build/_deps/kokkos-install-omp

cmake -B build/kokkoscomm-mpi-omp -G Ninja -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -DCMAKE_COLOR_DIAGNOSTICS=ON -DCMAKE_CXX_STANDARD=20 -DCMAKE_CXX_EXTENSIONS=OFF -DCMAKE_CXX_FLAGS='-Wall -Wextra -Wpedantic -Wmissing-include-dirs' -DKokkos_ROOT=build/_deps/kokkos-install-omp -DKokkosComm_ENABLE_MPI=ON -DKokkosComm_ENABLE_TESTS=ON -DKokkosComm_ENABLE_PERFTESTS=ON
cmake --build build/kokkoscomm-mpi-omp -j
ctest --test-dir build/kokkoscomm-mpi-omp --output-on-failure

[cedricchevalier19]

@dssgabriel, a related? openmpi issue: open-mpi/ompi#10017.