add slice_dim draft

paddle/framework/dim.h

@@ -401,5 +401,20 @@ HOSTDEVICE Dim<D> linear_to_dimension(int linear_index, Dim<D> extents) {
   return result;
 }
 
+template <int D, int S>
+Dim<D> slice(const Dim<S>& dim, int begin, int end) {
+  PADDLE_ENFORCE(begin < end,
+                 "Begin index must be less than end index in Dim slice.");
+  PADDLE_ENFORCE(begin >= 0 && end <= S && end - begin == D,
+                 "Index error occurs in Dim slice.");
+  if (begin > 0) {
+    return slice<D>(dim.tail, begin - 1, end - 1);
+  }
+  if (D == 1) {
+    return Dim<1>(dim.head);
+  }
+  return Dim<D>(dim.head, slice<D - 1>(dim.tail, 0, end - 1));
+}
+
 }  // namespace framework
 }  // namespace paddle