Add option to brute_force index to maintain reference to non-owning n…

…orms (#1865)

This makes the faiss integration substantially easier, since we can just use the existing norms that have already been calculated in GpuDistanceParams::vectorNorms - rather than require an owned copy that lives in the brute force index.

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

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

cpp/include/raft/neighbors/brute_force_types.hpp

@@ -66,11 +66,11 @@ struct index : ann::index {
   /** Dataset norms */
   [[nodiscard]] inline auto norms() const -> device_vector_view<const T, int64_t, row_major>
   {
-    return make_const_mdspan(norms_.value().view());
+    return norms_view_.value();
   }
 
   /** Whether ot not this index has dataset norms */
-  [[nodiscard]] inline bool has_norms() const noexcept { return norms_.has_value(); }
+  [[nodiscard]] inline bool has_norms() const noexcept { return norms_view_.has_value(); }
 
   [[nodiscard]] inline T metric_arg() const noexcept { return metric_arg_; }
 
@@ -102,10 +102,30 @@ struct index : ann::index {
       norms_(std::move(norms)),
       metric_arg_(metric_arg)
   {
+    if (norms_) { norms_view_ = make_const_mdspan(norms_.value().view()); }
     update_dataset(res, dataset);
     resource::sync_stream(res);
   }
 
+  /** Construct a brute force index from dataset
+   *
+   * This class stores a non-owning reference to the dataset and norms here.
+   * Having precomputed norms gives us a performance advantage at query time.
+   */
+  index(raft::resources const& res,
+        raft::device_matrix_view<const T, int64_t, row_major> dataset_view,
+        std::optional<raft::device_vector_view<const T, int64_t>> norms_view,
+        raft::distance::DistanceType metric,
+        T metric_arg = 0.0)
+    : ann::index(),
+      metric_(metric),
+      dataset_(make_device_matrix<T, int64_t>(res, 0, 0)),
+      dataset_view_(dataset_view),
+      norms_view_(norms_view),
+      metric_arg_(metric_arg)
+  {
+  }
+
  private:
   /**
    * Replace the dataset with a new dataset.
@@ -135,6 +155,7 @@ struct index : ann::index {
   raft::distance::DistanceType metric_;
   raft::device_matrix<T, int64_t, row_major> dataset_;
   std::optional<raft::device_vector<T, int64_t>> norms_;
+  std::optional<raft::device_vector_view<const T, int64_t>> norms_view_;
   raft::device_matrix_view<const T, int64_t, row_major> dataset_view_;
   T metric_arg_;
 };