Handling all identical vectors in UMAP smooth_knn_dist_kernel (#6904)

`smoot_knn_dist_kernel` just fails if all neighbors have zero distance (i.e. are identical vectors).

Instead of failing, set `rhos` and `sigmas` to specific values in such cases.

Closes #7017

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

Approvers:
  - Victor Lafargue (https://github.com/viclafargue)
  - Simon Adorf (https://github.com/csadorf)

URL: #6904

Author: jinsolp

cpp/src/umap/fuzzy_simpl_set/naive.cuh

@@ -80,8 +80,7 @@ static const float MIN_K_DIST_SCALE   = 1e-3;
  *
  */
 template <typename value_t, typename nnz_t, int TPB_X>
-CUML_KERNEL void smooth_knn_dist_kernel(bool* error_status,
-                                        const value_t* knn_dists,
+CUML_KERNEL void smooth_knn_dist_kernel(const value_t* knn_dists,
                                         int n,
                                         float mean_dist,
                                         value_t* sigmas,
@@ -122,7 +121,9 @@ CUML_KERNEL void smooth_knn_dist_kernel(bool* error_status,
     }
 
     if (start_nonzero == -1) {
-      *error_status = true;
+      // All distances are zero: identical neighbors
+      rhos[row]   = 0.0;
+      sigmas[row] = MIN_K_DIST_SCALE * mean_dist;  // e.g. 1e-5 * mean_dist
       return;
     }
 
@@ -272,17 +273,9 @@ void smooth_knn_dist(nnz_t n,
    * Smooth kNN distances to be continuous
    */
 
-  bool has_found_an_error = false;
-  rmm::device_scalar<bool> error_status(stream);
-  error_status.set_value_async(has_found_an_error, stream);
-
   smooth_knn_dist_kernel<value_t, nnz_t, TPB_X><<<grid, blk, 0, stream>>>(
-    error_status.data(), knn_dists, n, mean_dist, sigmas, rhos, n_neighbors, local_connectivity);
+    knn_dists, n, mean_dist, sigmas, rhos, n_neighbors, local_connectivity);
   RAFT_CUDA_TRY(cudaStreamSynchronize(stream));
-
-  has_found_an_error = error_status.value(stream);
-  RAFT_EXPECTS(!has_found_an_error,
-               "At least one row does not have any neighbor with non-zero distance.");
 }
 
 template <typename value_t, typename value_idx, typename nnz_t, int TPB_X>

python/cuml/cuml_accel_tests/upstream/umap/xfail-list.yaml

@@ -17,7 +17,6 @@
   - "umap.tests.test_umap_ops::test_umap_update"
   - "umap.tests.test_umap_ops::test_umap_update_large"
   - "umap.tests.test_umap_repeated_data::test_repeated_points_large_n"
-  - "umap.tests.test_umap_trustworthiness::test_umap_sparse_trustworthiness"
   - "umap.tests.test_umap_validation_params::test_umap_bad_hellinger_data"
   - "umap.tests.test_umap_validation_params::test_umap_bad_metrics"
   - "umap.tests.test_umap_validation_params::test_umap_bad_n_components"