fix lamb grad functor (#4163)

oneflow/user/kernels/model_update_kernel_util.h

@@ -101,7 +101,7 @@ struct LambGradFunctor {
         CastScaleRegularizeGradientFunctor<T, G>()(*model_diff, model_val, scale, l1, l2);
     const T next_m = beta1 * *m + (1 - beta1) * model_diff_t;
     const T next_v = beta2 * *v + (1 - beta2) * model_diff_t * model_diff_t;
-    *adam_diff = (next_m / (1 - *beta1_t)) / std::sqrt(next_v / (1 - *beta2_t) + epsilon);
+    *adam_diff = (next_m / (1 - *beta1_t)) / (std::sqrt(next_v / (1 - *beta2_t)) + epsilon);
     *m = next_m;
     *v = next_v;
   }