Fix bug in MG Neighborhood sampling (#4827)

This PR fixes a bug when computing the global start vertex labels

Authors:
  - Joseph Nke (https://github.com/jnke2016)

Approvers:
  - Alex Barghi (https://github.com/alexbarghi-nv)
  - Seunghwa Kang (https://github.com/seunghwak)
  - Naim (https://github.com/naimnv)
  - Chuck Hastings (https://github.com/ChuckHastings)

URL: #4827

cpp/src/c_api/neighbor_sampling.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022-2024, NVIDIA CORPORATION.
+ * Copyright (c) 2022-2025, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -880,7 +880,6 @@ struct neighbor_sampling_functor : public cugraph::c_api::abstract_functor {
                  handle_.get_stream());
 
       std::optional<rmm::device_uvector<label_t>> start_vertex_labels{std::nullopt};
-      std::optional<rmm::device_uvector<label_t>> local_label_to_comm_rank{std::nullopt};
       std::optional<rmm::device_uvector<label_t>> label_to_comm_rank{
         std::nullopt};  // global after allgatherv
 
@@ -932,12 +931,13 @@ struct neighbor_sampling_functor : public cugraph::c_api::abstract_functor {
             handle_.get_stream(),
             raft::device_span<label_t>{unique_labels.data(), unique_labels.size()});
 
-          (*local_label_to_comm_rank).resize(num_unique_labels, handle_.get_stream());
+          rmm::device_uvector<label_t> local_label_to_comm_rank(num_unique_labels,
+                                                                handle_.get_stream());
 
           cugraph::detail::scalar_fill(
             handle_.get_stream(),
-            (*local_label_to_comm_rank).begin(),  // This should be rename to rank
-            (*local_label_to_comm_rank).size(),
+            local_label_to_comm_rank.begin(),  // This should be rename to rank
+            local_label_to_comm_rank.size(),
             label_t{handle_.get_comms().get_rank()});
 
           // Perform allgather to get global_label_to_comm_rank_d_vector
@@ -948,11 +948,11 @@ struct neighbor_sampling_functor : public cugraph::c_api::abstract_functor {
           std::exclusive_scan(
             recvcounts.begin(), recvcounts.end(), displacements.begin(), size_t{0});
 
-          (*label_to_comm_rank)
-            .resize(displacements.back() + recvcounts.back(), handle_.get_stream());
+          label_to_comm_rank = rmm::device_uvector<label_t>(
+            displacements.back() + recvcounts.back(), handle_.get_stream());
 
           cugraph::device_allgatherv(handle_.get_comms(),
-                                     (*local_label_to_comm_rank).begin(),
+                                     local_label_to_comm_rank.begin(),
                                      (*label_to_comm_rank).begin(),
                                      recvcounts,
                                      displacements,