[BUG] raft::linalg::reduce won't compile when InType != OutType

[Nyrio]

Describe the bug
I am reducing a boolean matrix to an integer vector (cf code sample in the section below).
I only need coalescedReduction, but the reduce function instantiates stridedReduction too and this is where the problem happens, at line 150 of strided_reduction.cuh: the if statement is not determined at compile time (it's not a C++17 if constexpr) so it attempts to compile stridedSummationKernel even if the input and output types are different, in which case it can't compile.

Note that at the moment I solved the problem for my use case by calling directly coalescedReduction instead of reduce.

Steps/Code to reproduce bug
This is approximately what my code looks like:

bool* in;
int* out;
raft::linalg::reduce(
  out, in, N, N, 0, true, true, stream, false,
  [] __device__(bool value, int idx) {
    return static_cast<int>(value);
  },
  raft::Sum<int>(), raft::Nop<int>());

Compilation error:

include/raft/linalg/strided_reduction.cuh(152): error: no instance of function template "raft::linalg::stridedSummationKernel" matches the argument list
argument types are: (int *, const __nv_bool *, int, int, int, lambda [](__nv_bool, int)->int)
detected during: instantiation of "void raft::linalg::stridedReduction(OutType *, const InType *, IdxType, IdxType, OutType, cudaStream_t, __nv_bool, MainLambda, ReduceLambda, FinalLambda) [with InType=__nv_bool, OutType=int, IdxType=int, MainLambda=lambda [](__nv_bool, int)->int, ReduceLambda=raft::Sum<int>, FinalLambda=raft::Nop<int, int>]"

Expected behavior
The reduction should support different input and output types as suggested by the template parameters and the docs.

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[divyegala]

@Nyrio now that RAFT uses C++17, do you think you could submit a PR with a fix for this?

[Nyrio]

do you think you could submit a PR with a fix for this?

@divyegala Yes, I will.