Fix std::lock_guard use for gcc 14 support (#639)

The constructor of `std::lock_guard` in gcc 14 <mutex>, which is supported by CUDA 12.8, has the `nodiscard` attribute. This PR fixes the error occurring with gcc 14.

Authors:
  - tsuki (https://github.com/enp1s0)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)
  - Artem M. Chirkin (https://github.com/achirkin)

URL: #639

cpp/bench/ann/src/common/thread_pool.hpp

@@ -61,7 +61,7 @@ class fixed_thread_pool {
     finished_.store(true, std::memory_order_relaxed);
     for (unsigned i = 0; i < threads_.size(); ++i) {
       auto& task = tasks_[i];
-      std::lock_guard<std::mutex>(task.mtx);
+      std::lock_guard lock(task.mtx);
 
       task.cv.notify_one();
       threads_[i].join();

cpp/src/neighbors/iface/iface.hpp

@@ -36,7 +36,7 @@ void build(const raft::device_resources& handle,
            const cuvs::neighbors::index_params* index_params,
            raft::mdspan<const T, matrix_extent<int64_t>, row_major, Accessor> index_dataset)
 {
-  std::lock_guard(*interface.mutex_);
+  std::lock_guard lock(*interface.mutex_);
 
   if constexpr (std::is_same<AnnIndexType, ivf_flat::index<T, IdxT>>::value) {
     auto idx = cuvs::neighbors::ivf_flat::build(
@@ -62,7 +62,7 @@ void extend(
   std::optional<raft::mdspan<const IdxT, vector_extent<int64_t>, layout_c_contiguous, Accessor2>>
     new_indices)
 {
-  std::lock_guard(*interface.mutex_);
+  std::lock_guard lock(*interface.mutex_);
 
   if constexpr (std::is_same<AnnIndexType, ivf_flat::index<T, IdxT>>::value) {
     auto idx =
@@ -141,7 +141,7 @@ void serialize(const raft::device_resources& handle,
                const cuvs::neighbors::iface<AnnIndexType, T, IdxT>& interface,
                std::ostream& os)
 {
-  std::lock_guard(*interface.mutex_);
+  std::lock_guard lock(*interface.mutex_);
 
   if constexpr (std::is_same<AnnIndexType, ivf_flat::index<T, IdxT>>::value) {
     ivf_flat::serialize(handle, os, interface.index_.value());
@@ -157,7 +157,7 @@ void deserialize(const raft::device_resources& handle,
                  cuvs::neighbors::iface<AnnIndexType, T, IdxT>& interface,
                  std::istream& is)
 {
-  std::lock_guard(*interface.mutex_);
+  std::lock_guard lock(*interface.mutex_);
 
   if constexpr (std::is_same<AnnIndexType, ivf_flat::index<T, IdxT>>::value) {
     ivf_flat::index<T, IdxT> idx(handle);
@@ -179,7 +179,7 @@ void deserialize(const raft::device_resources& handle,
                  cuvs::neighbors::iface<AnnIndexType, T, IdxT>& interface,
                  const std::string& filename)
 {
-  std::lock_guard(*interface.mutex_);
+  std::lock_guard lock(*interface.mutex_);
 
   std::ifstream is(filename, std::ios::in | std::ios::binary);
   if (!is) { RAFT_FAIL("Cannot open file %s", filename.c_str()); }