Revert "Remove the remnant code (test=develop)"

This reverts commit be50670.

Author: yihuaxu

paddle/fluid/operators/stack_op.h

@@ -72,6 +72,25 @@ class StackOpMaker : public framework::OpProtoAndCheckerMaker {
   }
 };
 
+template <typename VecXType, typename T>
+struct StackFunctor {
+  HOSTDEVICE StackFunctor(const VecXType &x, T *y, int n, int post)
+      : x_(x), y_(y), n_(n), post_(post) {}
+
+  HOSTDEVICE void operator()(int idx) {
+    int i = idx / (n_ * post_);
+    int which_x = idx / post_ - i * n_;
+    int x_index = i * post_ + idx % post_;
+    y_[idx] = x_[which_x][x_index];
+  }
+
+ private:
+  VecXType x_;
+  T *y_;
+  int n_;
+  int post_;
+};
+
 template <typename VecDxType, typename T>
 struct StackGradFunctor {
   HOSTDEVICE StackGradFunctor(const VecDxType &dx, const T *dy, int n, int post)
@@ -91,6 +110,14 @@ struct StackGradFunctor {
   int post_;
 };
 
+template <typename DeviceContext, typename VecXType, typename T>
+static inline void StackFunctorForRange(const DeviceContext &ctx,
+                                        const VecXType &x, T *y, int total_num,
+                                        int n, int post) {
+  platform::ForRange<DeviceContext> for_range(ctx, total_num);
+  for_range(StackFunctor<VecXType, T>(x, y, n, post));
+}
+
 template <typename DeviceContext, typename VecDxType, typename T>
 static inline void StackGradFunctorForRange(const DeviceContext &ctx,
                                             const VecDxType &dx, const T *dy,