ANN_BENCH: Fix segfault in CAGRA wrapper when moving its graph

When the graph in CAGRA wrapper needs to be relocated to a different memory resource, the old one is destructed before the data is copied. This leads to a segfault during copy.
The fix is to retain the old graph pointer until the data is copied.

cpp/bench/ann/src/cuvs/cuvs_cagra_wrapper.h

@@ -250,13 +250,16 @@ void cuvs_cagra<T, IdxT>::set_search_param(const search_param_base& param,
     RAFT_LOG_DEBUG("moving graph to new memory space: %s", allocator_to_string(graph_mem_).c_str());
     // We create a new graph and copy to it from existing graph
     auto mr = get_mr(graph_mem_);
-    *graph_ = raft::make_device_mdarray<IdxT, int64_t>(
-      handle_, mr, raft::make_extents<int64_t>(index_->graph().extent(0), index_->graph_degree()));
 
-    raft::copy(graph_->data_handle(),
-               index_->graph().data_handle(),
-               index_->graph().size(),
+    // Create a new graph, then copy, and __only then__ replace the shared pointer.
+    // We shouldn't use index_->graph() here, because it may be just a view of graph_.
+    auto new_graph = raft::make_device_mdarray<IdxT, int64_t>(handle_, mr, graph_->extents());
+    raft::copy(new_graph.data_handle(),
+               graph_->data_handle(),
+               graph_->size(),
                raft::resource::get_cuda_stream(handle_));
+    raft::resource::sync_stream(handle_);
+    *graph_ = std::move(new_graph);
 
     // NB: update_graph() only stores a view in the index. We need to keep the graph object alive.
     index_->update_graph(handle_, make_const_mdspan(graph_->view()));