fix type conversion problem in rsqrt functor

paddle/fluid/operators/activation_op.h

@@ -477,7 +477,7 @@ struct RsqrtGradFunctor : 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>(-0.5) * dout * out.pow(3);
+    dx.device(d) = static_cast<T>(-0.5) * dout * out * out * out;
   }
 };