IVF-Flat: fix search batching (#1764)

Fix the cluster probes (coarse_index) not being advanced when batching.

Thanks @tarang-jain for the precise reproducer.

Closes: #1756

Authors:
  - Artem M. Chirkin (https://github.com/achirkin)

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

URL: #1764

cpp/include/raft/neighbors/detail/ivf_flat_interleaved_scan-inl.cuh

@@ -884,6 +884,7 @@ void launch_kernel(Lambda lambda,
     queries += grid_dim_y * index.dim();
     neighbors += grid_dim_y * grid_dim_x * k;
     distances += grid_dim_y * grid_dim_x * k;
+    coarse_index += grid_dim_y * n_probes;
   }
 }
 

cpp/test/neighbors/ann_ivf_flat.cuh

@@ -497,6 +497,11 @@ const std::vector<AnnIvfFlatInputs<int64_t>> inputs = {
    raft::matrix::detail::select::warpsort::kMaxCapacity * 4,
    raft::matrix::detail::select::warpsort::kMaxCapacity * 4,
    raft::distance::DistanceType::InnerProduct,
-   false}};
+   false},
+
+  // The following two test cases should show very similar recall.
+  // num_queries, num_db_vecs, dim, k, nprobe, nlist, metric, adaptive_centers
+  {20000, 8712, 3, 10, 51, 66, raft::distance::DistanceType::L2Expanded, false},
+  {100000, 8712, 3, 10, 51, 66, raft::distance::DistanceType::L2Expanded, false}};
 
 }  // namespace raft::neighbors::ivf_flat