use eval to improve performance, test=develop (#25459)

paddle/fluid/operators/dist_op.h

@@ -176,15 +176,27 @@ static void DistGradFunction(const framework::ExecutionContext& context) {
   } else if (p == INFINITY || p == -INFINITY) {
     // p=inf or -inf, Lp-norm = |z_i|, the j-th element of dz tends to 0 if
     // j!=i, or equals to sign(z_i) * dout if j=i.
-    grad_t.device(place) =
-        (x_minux_y_abs == out_t.broadcast(out_bcast_dims)).template cast<T>() *
+    if (platform::is_cpu_place(context.GetPlace())) {
+      grad_t.device(place) = (x_minux_y_abs == out_t.broadcast(out_bcast_dims))
+                                 .template cast<T>() *
+                             sign.eval() * out_grad_t.broadcast(out_bcast_dims);
+    } else {
+      grad_t.device(place) = (x_minux_y_abs == out_t.broadcast(out_bcast_dims))
+                                 .template cast<T>() *
                              sign * out_grad_t.broadcast(out_bcast_dims);
+    }
   } else {
     // dz = pow(abs(x-y)/out, p-1) * sign(x-y) * dout
+    if (platform::is_cpu_place(context.GetPlace())) {
+      grad_t.device(place) =
+          (x_minux_y_abs / out_t.broadcast(out_bcast_dims)).pow(p - 1) *
+          sign.eval() * out_grad_t.broadcast(out_bcast_dims);
+    } else {
       grad_t.device(place) =
           (x_minux_y_abs / out_t.broadcast(out_bcast_dims)).pow(p - 1) * sign *
           out_grad_t.broadcast(out_bcast_dims);
     }
+  }
 
   Eigen::DSizes<int, Rank * 2> x_reshape_dims;
   Eigen::DSizes<int, Rank * 2> y_reshape_dims;