refine ZeroGradFunctor in activation_op.h

paddle/fluid/operators/activation_op.h

@@ -353,7 +353,7 @@ struct ZeroGradFunctor : public BaseActivationFunctor<T> {
   template <typename Device, typename X, typename Out, typename dOut,
             typename dX>
   void operator()(Device d, X x, Out out, dOut dout, dX dx) const {
-    dx.device(d) = static_cast<T>(0) / out;
+    dx.device(d) = out.constant(static_cast<T>(0));
   }
 };