reserve vector space before loop in top-k

paddle/fluid/operators/top_k_op.h

@@ -60,6 +60,7 @@ class TopkKernel : public framework::OpKernel<T> {
 #endif
     for (size_t i = 0; i < row; i++) {
       std::vector<std::pair<T, size_t>> vec;
+      vec.reserve(col);
       for (size_t j = 0; j < col; j++) {
         vec.push_back(std::pair<T, size_t>(eg_input(i, j), j));
       }