Use slicing kernel to copy distances inside NN Descent (#2380)

This make use of raft's slicing kernel within NN Descent build.
I found that my previous implementation was inefficient (merged in [this PR](#2345)).

### Improvements
Time to call NN Descent `build()` with `return_distances=True` before and after using this kernel
| Dataset | Before |After|
| ------------- | ------------- |---|
| mnist (60000, 784)  | 1550ms  | 1020ms|
| sift (1M, 128)  | 11342ms  |5546ms|
| gist (1M, 960)  | 13508ms |9278ms|

Authors:
  - Jinsol Park (https://github.com/jinsolp)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)
  - Micka (https://github.com/lowener)

URL: #2380

cpp/include/raft/neighbors/detail/nn_descent.cuh

@@ -19,11 +19,14 @@
 #include "../nn_descent_types.hpp"
 
 #include <raft/core/device_mdarray.hpp>
+#include <raft/core/device_mdspan.hpp>
 #include <raft/core/error.hpp>
 #include <raft/core/host_mdarray.hpp>
+#include <raft/core/mdspan.hpp>
 #include <raft/core/operators.hpp>
 #include <raft/core/resource/cuda_stream.hpp>
 #include <raft/core/resources.hpp>
+#include <raft/matrix/slice.cuh>
 #include <raft/neighbors/detail/cagra/device_common.hpp>
 #include <raft/spatial/knn/detail/ann_utils.cuh>
 #include <raft/util/arch.cuh>  // raft::util::arch::SM_*
@@ -1365,12 +1368,22 @@ void GNND<Data_t, Index_t, epilogue_op>::build(Data_t* data,
   static_assert(sizeof(decltype(*(graph_.h_dists.data_handle()))) >= sizeof(Index_t));
 
   if (return_distances) {
-    for (size_t i = 0; i < (size_t)nrow_; i++) {
-      raft::copy(output_distances + i * build_config_.output_graph_degree,
-                 graph_.h_dists.data_handle() + i * build_config_.node_degree,
-                 build_config_.output_graph_degree,
-                 raft::resource::get_cuda_stream(res));
-    }
+    auto graph_d_dists = raft::make_device_matrix<DistData_t, int64_t, raft::row_major>(
+      res, nrow_, build_config_.node_degree);
+    raft::copy(graph_d_dists.data_handle(),
+               graph_.h_dists.data_handle(),
+               nrow_ * build_config_.node_degree,
+               raft::resource::get_cuda_stream(res));
+
+    auto output_dist_view = raft::make_device_matrix_view<DistData_t, int64_t, raft::row_major>(
+      output_distances, nrow_, build_config_.output_graph_degree);
+
+    raft::matrix::slice_coordinates coords{static_cast<int64_t>(0),
+                                           static_cast<int64_t>(0),
+                                           static_cast<int64_t>(nrow_),
+                                           static_cast<int64_t>(build_config_.output_graph_degree)};
+    raft::matrix::slice<DistData_t, int64_t, raft::row_major>(
+      res, raft::make_const_mdspan(graph_d_dists.view()), output_dist_view, coords);
   }
 
   Index_t* graph_shrink_buffer = (Index_t*)graph_.h_dists.data_handle();