Opt topk (#37256)

topk中有cub和手写kernel两种实现，而cub是通过排序来获取topk，通过多组数据发现只有当input_width>=128且k超过input_width 75%的时候性能会比手写的更好。

paddle/fluid/operators/top_k_v2_op.cu

@@ -83,7 +83,9 @@ class TopkV2OpCUDAKernel : public framework::OpKernel<T> {
 
       if (k > input_width) k = input_width;
 
-      if ((input_width <= 1024 || k >= 128 || k == input_width)) {
+      // The conclusion is drawn from the data through multiple sets of
+      // statistics
+      if (input_width >= 128 && k >= input_width * 0.75) {
         if (SortTopk<T>(dev_ctx, input, input_width, input_height, k, output,
                         indices, largest)) {
           // Successed, return.
@@ -159,8 +161,9 @@ class TopkV2OpCUDAKernel : public framework::OpKernel<T> {
 
       if (k > input_width) k = input_width;
 
-      if (((input_width <= 1024 && input_height <= 2048) || k >= 128 ||
-           k == input_width)) {
+      // The conclusion is drawn from the data through multiple sets of
+      // statistics
+      if (input_width >= 128 && k >= input_width * 0.75) {
         if (SortTopk<T>(dev_ctx, &trans_input, input_width, input_height, k,
                         &trans_out, &trans_ind, largest)) {
           // last step, tranpose back the indices and output