sort: single API entry point for basic overloads and move code to impl

[fnrizzi]

Another step towards the refactoring and a stepping stone for adding the custom comparator in a separate PR.
This PR moves code from the public API to the impl such that these lower-level functions can be reused if needed, and to have a single public API entry point where it is easier to reason from.

Before

template <class ExecutionSpace, class DataType, class... Properties>
std::enable_if_t<(Kokkos::is_execution_space<ExecutionSpace>::value) &&
                 (!SpaceAccessibility<
                     HostSpace, typename Kokkos::View<DataType, Properties...>::
                                    memory_space>::accessible)>
sort(const ExecutionSpace& exec, const Kokkos::View<DataType, Properties...>& view){
  // call binsort
}

#if defined(KOKKOS_ENABLE_ONEDPL)
template <class DataType, class... Properties>
void sort(const Experimental::SYCL& space, const Kokkos::View<DataType, Properties...>& view)
{
  // sort via onedpl
}
#endif

template <class ExecutionSpace, class DataType, class... Properties>
std::enable_if_t<(Kokkos::is_execution_space<ExecutionSpace>::value) &&
                 (SpaceAccessibility<
                     HostSpace, typename Kokkos::View<DataType, Properties...>::
                                    memory_space>::accessible)>
sort(const ExecutionSpace&, const Kokkos::View<DataType, Properties...>& view)
{
  // call std sort
}

#if defined(KOKKOS_ENABLE_CUDA)
template <class DataType, class... Properties>
void sort(const Cuda& space, const Kokkos::View<DataType, Properties...>& view)
{
  // call thrust 
}
#endif

template <class ViewType>
void sort(ViewType const& view) {
  // call overload with view's exespace
}

With this PR

template <class ExecutionSpace, class DataType, class... Properties>
void sort([[maybe_unused]] const ExecutionSpace& exec,
          const Kokkos::View<DataType, Properties...>& view) 
{
  // constraints ...
  // ...
  // for now this is called for OpenMP, Threads, Serial and HPX
  if constexpr (Impl::better_off_calling_std_sort_v<ExecutionSpace>){
    auto first = ::Kokkos::Experimental::begin(view);
    auto last  = ::Kokkos::Experimental::end(view);
    std::sort(first, last);
  } else {
    Impl::sort_device_view_without_comparator(exec, view);
  }

template <class DataType, class... Properties>
void sort(const Kokkos::View<DataType, Properties...>& view) {
  // call overload with view's exespace
}

whre Impl::sort_device_view_without_comparator(exec, view); takes care of dispatching things to maintain same behavior as before but this is inside the impl namespace:

namespace Impl{

#if defined(KOKKOS_ENABLE_CUDA)
template <class DataType, class... Properties>
void sort_device_view_without_comparator(const Cuda& exec, 
                                         const Kokkos::View<DataType, Properties...>& view) {
  sort_cudathrust(exec, view);
}
#endif

#if defined(KOKKOS_ENABLE_ONEDPL)
template <class DataType, class... Properties>
void sort_device_view_without_comparator(const Experimental::SYCL& exec,
                                         const Kokkos::View<DataType, Properties...>& view) { 
  sort_onedpl(exec, view);
}
#endif

// fallback case
template <class ExecutionSpace, class DataType, class... Properties>
std::enable_if_t<Kokkos::is_execution_space<ExecutionSpace>::value>
sort_device_view_without_comparator(const ExecutionSpace& exec,
                                     const Kokkos::View<DataType, Properties...>& view) {
  sort_via_binsort(exec, view);
}

} //end namespace Impl

[fnrizzi]

ci failure unrelated:

The following tests FAILED:
	 45 - Kokkos_UnitTest_Random (Failed)