Fix is_min_close (#1419)

Correlation and Cosine distance both return (1 - similarity) in the pairwise distances apis, meaning that is_min_close is returning the wrong sort order for them. Fix.

Authors:
  - Ben Frederickson (https://github.com/benfred)

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

URL: #1419

cpp/include/raft/distance/distance_types.hpp

@@ -74,8 +74,6 @@ inline bool is_min_close(DistanceType metric)
   bool select_min;
   switch (metric) {
     case DistanceType::InnerProduct:
-    case DistanceType::CosineExpanded:
-    case DistanceType::CorrelationExpanded:
       // Similarity metrics have the opposite meaning, i.e. nearest neighbors are those with larger
       // similarity (See the same logic at cpp/include/raft/sparse/spatial/detail/knn.cuh:362
       // {perform_k_selection})

cpp/include/raft/sparse/neighbors/detail/knn.cuh

@@ -355,8 +355,7 @@ class sparse_knn_t {
     // want to adjust k.
     value_idx n_neighbors = std::min(static_cast<value_idx>(k), batch_cols);
 
-    bool ascending = true;
-    if (metric == raft::distance::DistanceType::InnerProduct) ascending = false;
+    bool ascending = raft::distance::is_min_close(metric);
 
     // kernel to slice first (min) k cols and copy into batched merge buffer
     raft::spatial::knn::select_k(batch_dists,
@@ -425,4 +424,4 @@ class sparse_knn_t {
   raft::device_resources const& handle;
 };
 
-};  // namespace raft::sparse::neighbors::detail
+};  // namespace raft::sparse::neighbors::detail