Fix index overflow in edge cases of CAGRA graph optimize (#435)

Force `input_graph_degree`, `output_graph_degree`, and `graph_size` variables to `uint64_t`.
Before the PR, they've been `uint32_t`, and the product of them could overflow. This would lead to `cudaMemsetAsync` not filling in a large fraction of the graph.

It's not known whether this bug has surfaced for anyone until now, but it's better to be safe than sorry.
The change shouldn't have any impact on performance.

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

Approvers:
  - Micka (https://github.com/lowener)

URL: #435

cpp/src/neighbors/detail/cagra/graph_core.cuh

@@ -1076,11 +1076,11 @@ void optimize(
                "Each input array is expected to have the same number of rows");
   RAFT_EXPECTS(new_graph.extent(1) <= knn_graph.extent(1),
                "output graph cannot have more columns than input graph");
-  const uint32_t input_graph_degree  = knn_graph.extent(1);
-  const uint32_t output_graph_degree = new_graph.extent(1);
+  const uint64_t input_graph_degree  = knn_graph.extent(1);
+  const uint64_t output_graph_degree = new_graph.extent(1);
+  const uint64_t graph_size          = new_graph.extent(0);
   auto input_graph_ptr               = knn_graph.data_handle();
   auto output_graph_ptr              = new_graph.data_handle();
-  const IdxT graph_size              = new_graph.extent(0);
 
   // MST optimization
   auto mst_graph_num_edges     = raft::make_host_vector<uint32_t, int64_t>(graph_size);
@@ -1148,7 +1148,7 @@ void optimize(
     constexpr int MAX_DEGREE = 1024;
     if (input_graph_degree > MAX_DEGREE) {
       RAFT_FAIL(
-        "The degree of input knn graph is too large (%u). "
+        "The degree of input knn graph is too large (%zu). "
         "It must be equal to or smaller than %d.",
         input_graph_degree,
         1024);
@@ -1217,11 +1217,12 @@ void optimize(
         assert(next_num_detour != std::numeric_limits<uint32_t>::max());
         num_detour = next_num_detour;
       }
-      RAFT_EXPECTS(pk == output_graph_degree,
-                   "Couldn't find the output_graph_degree (%u) smallest detourable count nodes for "
-                   "node %lu in the rank-based node reranking process",
-                   output_graph_degree,
-                   static_cast<uint64_t>(i));
+      RAFT_EXPECTS(
+        pk == output_graph_degree,
+        "Couldn't find the output_graph_degree (%lu) smallest detourable count nodes for "
+        "node %lu in the rank-based node reranking process",
+        output_graph_degree,
+        i);
     }
 
     const double time_prune_end = cur_time();
@@ -1317,7 +1318,7 @@ void optimize(
       uint32_t kf       = 0;
       uint32_t k        = mst_graph_num_edges_ptr[i];
 
-      const uint64_t num_protected_edges = max(k, output_graph_degree / 2);
+      const auto num_protected_edges = std::max<uint64_t>(k, output_graph_degree / 2);
       assert(num_protected_edges <= output_graph_degree);
       if (num_protected_edges == output_graph_degree) continue;
 
@@ -1342,7 +1343,7 @@ void optimize(
       assert(kf <= output_graph_degree);
 
       // Replace some edges of the output graph with edges of the reverse graph.
-      uint32_t kr = std::min(rev_graph_count.data_handle()[i], output_graph_degree);
+      auto kr = std::min<uint32_t>(rev_graph_count.data_handle()[i], output_graph_degree);
       while (kr) {
         kr -= 1;
         if (my_rev_graph[kr] < graph_size) {