add a sync after flush cache when doing benchmark

Author: jianyizh

python/triton/testing.py

@@ -204,6 +204,8 @@ def do_bench(fn, warmup=25, rep=100, grad_to_none=None, quantiles=None, return_m
                 x.grad = None
         # we clear the L2 cache before each run
         cache.zero_()
+        if USE_WALL_TIME:
+            di.synchronize()
         # record time of `fn`
         start_event[i].record()
         fn()