Use HNSW GPU hierarchy by default

Author: julianmi

cpp/include/cuvs/neighbors/hnsw.hpp

@@ -40,7 +40,7 @@ enum class HnswHierarchy {
 
 struct index_params : cuvs::neighbors::index_params {
   /** Hierarchy build type for HNSW index when converting from CAGRA index */
-  HnswHierarchy hierarchy = HnswHierarchy::NONE;
+  HnswHierarchy hierarchy = HnswHierarchy::GPU;
   /** Size of the candidate list during hierarchy construction when hierarchy is `CPU`*/
   int ef_construction = 200;
   /** Number of host threads to use to construct hierarchy when hierarchy is `CPU` or `GPU`.

examples/cpp/src/cagra_hnsw_ace_example.cu

@@ -81,7 +81,8 @@ void cagra_build_search_ace(raft::device_resources const& dev_resources,
   // descriptor For in-memory indices: creates HNSW index in memory
   std::cout << "Converting CAGRA index to HNSW" << std::endl;
   hnsw::index_params hnsw_params;
-  auto hnsw_index = hnsw::from_cagra(dev_resources, hnsw_params, index);
+  hnsw_params.hierarchy = hnsw::HnswHierarchy::GPU;  // Offload hierarchy construction to GPU
+  auto hnsw_index       = hnsw::from_cagra(dev_resources, hnsw_params, index);
 
   // HNSW search requires host matrices
   auto queries_host = raft::make_host_matrix<float, int64_t>(n_queries, queries.extent(1));