Add KOKKOS_FUNCTION macro to Details::Sink::operator()

[Shihab-Shahriar]

While compiling ArborX for cuda, I had this warning:

warning #20014-D: calling a __host__ function from a __host__ __device__ function is not allowed
        out(i);
        ^
          detected during:
            instantiation of "void ExcludeSelfCollision::operator()(const Predicate &, int, const OutputFunctor &) const [with Predicate=ArborX::PredicateWithAttachment<ArborX::Intersects<ArborX::Sphere>, int>, OutputFunctor=ArborX::Details::Sink<int>]" at line 3261 of /usr/include/c++/11/type_traits

This small change got rid of that.

The ExcludeSelfCollision callback is defined as:

struct ExcludeSelfCollision
{
  template <class Predicate, class OutputFunctor>
  KOKKOS_FUNCTION void operator()(Predicate const &predicate, int i,
                                  OutputFunctor const &out) const
  {
    int const j = ArborX::getData(predicate);
    if (i != j)
    {
      out(i);
    }
  }
};

[aprokop]

@Shihab-Shahriar Thank you for the contribution.

We get this warnings in our testing too, but we don't catch them in CI due to #272. Confirmed that with this patch, the warnings disappeared from the CI. Also confirmed that we had this warning for at least few weeks, can't trace further back as CI logs are gone.

Perhaps, this started happening when switching to invoke_result in the Callbacks file in August.