Upgrade clang-tidy on CI. (#5469)

* Correct all clang-tidy errors.
* Upgrade clang-tidy to 10 on CI.

Co-authored-by: Hyunsu Cho <chohyu01@cs.washington.edu>

src/metric/elementwise_metric.cu

@@ -319,7 +319,7 @@ struct EvalTweedieNLogLik {
  */
 template<typename Policy>
 struct EvalEWiseBase : public Metric {
-  EvalEWiseBase() : policy_{}, reducer_{policy_} {}
+  EvalEWiseBase() = default;
   explicit EvalEWiseBase(char const* policy_param) :
     policy_{policy_param}, reducer_{policy_} {}
 
@@ -351,8 +351,7 @@ struct EvalEWiseBase : public Metric {
 
  private:
   Policy policy_;
-
-  ElementWiseMetricsReduction<Policy> reducer_;
+  ElementWiseMetricsReduction<Policy> reducer_{policy_};
 };
 
 XGBOOST_REGISTER_METRIC(RMSE, "rmse")

src/metric/metric_common.h

@@ -62,11 +62,11 @@ struct EvalRankConfig {
 };
 
 class PackedReduceResult {
-  double residue_sum_;
-  double weights_sum_;
+  double residue_sum_ { 0 };
+  double weights_sum_ { 0 };
 
  public:
-  XGBOOST_DEVICE PackedReduceResult() : residue_sum_{0}, weights_sum_{0} {}
+  XGBOOST_DEVICE PackedReduceResult() {}  // NOLINT
   XGBOOST_DEVICE PackedReduceResult(double residue, double weight)
       : residue_sum_{residue}, weights_sum_{weight} {}
 

src/metric/rank_metric.cu

@@ -135,7 +135,6 @@ struct EvalNDCGGpu {
     dh::caching_device_vector<double> &dcgs(*dcgptr);
     // Group info on device
     const auto &dgroups = pred_sorter.GetGroupsSpan();
-    const auto ngroups = pred_sorter.GetNumGroups();
     const auto &dgroup_idx = pred_sorter.GetGroupSegmentsSpan();
 
     // First, determine non zero labels in the dataset individually

src/metric/survival_metric.cc

@@ -40,11 +40,11 @@ struct EvalAFT : public Metric {
   void SaveConfig(Json* p_out) const override {
     auto& out = *p_out;
     out["name"] = String(this->Name());
-    out["aft_loss_param"] = toJson(param_);
+    out["aft_loss_param"] = ToJson(param_);
   }
 
   void LoadConfig(Json const& in) override {
-    fromJson(in["aft_loss_param"], &param_);
+    FromJson(in["aft_loss_param"], &param_);
   }
 
   bst_float Eval(const HostDeviceVector<bst_float> &preds,