fix dp save issue

Author: yuankuns

examples/sycl/sdpa_bwd/sdpa_backward.cpp

@@ -513,6 +513,7 @@ dq_dk_dv_1colblock(Trait &trait, Param<typename Trait::DType> &param,
         sycl::group_barrier(group);
 
         copy(tilesavedP, tdPrdPl, tdPgdP); // save dP to buffer after P used by dV
+        sycl::group_barrier(group);
 
         if (n_block > 0)
             copy(tileloaddQ, tdQgdQ, tdQrdQ);