Planning: fixed the initialization bug in dp_st_graph (#2753)

modules/planning/tasks/dp_st_speed/dp_st_graph.cc

@@ -69,6 +69,10 @@ DpStGraph::DpStGraph(const StGraphData& st_graph_data,
   dp_st_speed_config_.set_total_path_length(
       std::fmin(dp_st_speed_config_.total_path_length(),
                 st_graph_data_.path_data_length()));
+  unit_s_ = dp_st_speed_config_.total_path_length() /
+            dp_st_speed_config_.matrix_dimension_s();
+  unit_t_ = dp_st_speed_config_.total_time() /
+            dp_st_speed_config_.matrix_dimension_t();
 }
 
 Status DpStGraph::Search(SpeedData* const speed_data) {
@@ -131,9 +135,6 @@ Status DpStGraph::Search(SpeedData* const speed_data) {
 Status DpStGraph::InitCostTable() {
   uint32_t dim_s = dp_st_speed_config_.matrix_dimension_s();
   uint32_t dim_t = dp_st_speed_config_.matrix_dimension_t();
-  unit_s_ = dp_st_speed_config_.total_path_length() / dim_s;
-  unit_t_ = dp_st_speed_config_.total_time() /
-            dp_st_speed_config_.matrix_dimension_t();
   DCHECK_GT(dim_s, 2);
   DCHECK_GT(dim_t, 2);
   cost_table_ = std::vector<std::vector<StGraphPoint>>(