Fix race condition in custom_inclusive_scan_over_group

By returning data from `local_mem_acc` after the group barrier, if memory is later overwritten, a race condition follows, which was especially obvious on CPU

Now the value is stored in variable before the barrier and then returned

Author: ndgrigorian

dpctl/tensor/libtensor/include/utils/sycl_utils.hpp

@@ -172,9 +172,10 @@ T custom_inclusive_scan_over_group(const GroupT &wg,
         }
     }
 
+    T accumulated_local_val = local_mem_acc[local_id];
     sycl::group_barrier(wg, sycl::memory_scope::work_group);
 
-    return local_mem_acc[local_id];
+    return accumulated_local_val;
 }
 
 // Reduction functors