speed up stack_op

paddle/fluid/operators/stack_op.h

@@ -150,12 +150,7 @@ class StackKernel : public framework::OpKernel<T> {
     int total_num = pre * n * post;
 
     auto &dev_ctx = ctx.template device_context<DeviceContext>();
-    constexpr auto kMaxThreshold = 16;
-    if (std::is_same<DeviceContext, platform::CPUDeviceContext>::value ||
-        n > kMaxThreshold) {
 #ifdef __NVCC__
-      VLOG(10) << "Stack more than " << kMaxThreshold
-               << " tensors on GPU may be slow.";
     thrust::device_vector<const T *> device_x_vec(x_datas);
     auto x_data_arr = device_x_vec.data().get();
 #else
@@ -168,14 +163,6 @@ class StackKernel : public framework::OpKernel<T> {
     dev_ctx.Wait();
 #endif
   }
-#ifdef __NVCC__
-    else {  // NOLINT
-      framework::Array<const T *, kMaxThreshold> x_data_arr;
-      for (int i = 0; i < n; ++i) x_data_arr[i] = x_datas[i];
-      StackFunctorForRange(dev_ctx, x_data_arr, y_data, total_num, n, post);
-    }
-#endif
-  }
 };
 
 class StackOpGrad : public framework::OperatorWithKernel {
@@ -244,34 +231,19 @@ class StackGradKernel : public framework::OpKernel<T> {
     int post = total_num / (n * pre);
 
     auto &dev_ctx = ctx.template device_context<DeviceContext>();
-    constexpr auto kMaxThreshold = 16;
-    if (std::is_same<DeviceContext, platform::CPUDeviceContext>::value ||
-        n > kMaxThreshold) {
 #ifdef __NVCC__
-      VLOG(10) << "Stack more than " << kMaxThreshold
-               << " tensors on GPU may be slow.";
     thrust::device_vector<T *> device_dx_vec(dx_datas);
     auto dx_data_arr = device_dx_vec.data().get();
 #else
     auto dx_data_arr = dx_datas.data();
 #endif
-      StackGradFunctorForRange(dev_ctx, dx_data_arr, dy_data, total_num, n,
-                               post);
+    StackGradFunctorForRange(dev_ctx, dx_data_arr, dy_data, total_num, n, post);
 #ifdef __NVCC__
     // Wait() must be called because device_dx_vec may be destructed before
     // kernel ends
     dev_ctx.Wait();
 #endif
   }
-#ifdef __NVCC__
-    else {  // NOLINT
-      framework::Array<T *, kMaxThreshold> dx_data_arr;
-      for (int i = 0; i < n; ++i) dx_data_arr[i] = dx_datas[i];
-      StackGradFunctorForRange(dev_ctx, dx_data_arr, dy_data, total_num, n,
-                               post);
-    }
-#endif
-  }
 };
 
 }  // namespace operators