Fix weighted samples in multi-class AUC. (#7300)

tests/cpp/helpers.h

@@ -86,7 +86,7 @@ void CheckRankingObjFunction(std::unique_ptr<xgboost::ObjFunction> const& obj,
 
 xgboost::bst_float GetMetricEval(
   xgboost::Metric * metric,
-  xgboost::HostDeviceVector<xgboost::bst_float> preds,
+  xgboost::HostDeviceVector<xgboost::bst_float> const& preds,
   std::vector<xgboost::bst_float> labels,
   std::vector<xgboost::bst_float> weights = std::vector<xgboost::bst_float>(),
   std::vector<xgboost::bst_uint> groups = std::vector<xgboost::bst_uint>());