Add warp sync for Cuda parallel reduce

compute-sanitizer --tool=racecheck discovered a potential racecondition for Cuda parallel reductions (using range policy) where data was being updated on a single thread inside a warp, but the warp was not being synchronized before being read.

core/src/Cuda/Kokkos_Cuda_Parallel_Range.hpp

@@ -243,6 +243,12 @@ class ParallelReduce<CombinedFunctorReducerType, Kokkos::RangePolicy<Traits...>,
 
       if (CudaTraits::WarpSize < word_count.value) {
         __syncthreads();
+      } else if (word_count.value > 1) {
+        // Inside cuda_single_inter_block_reduce_scan() above, shared[i] below
+        // might have been updated by a single thread within a warp without
+        // synchronization afterwards. Synchronize threads within warp to avoid
+        // potential racecondition.
+        __syncwarp(0xffffffff);
       }
 
       for (unsigned i = threadIdx.y; i < word_count.value; i += blockDim.y) {