Pass obj info by reference instead of by value. (#8889)

- Pass obj info into tree updater as const pointer.

This way we don't have to initialize the learner model param before configuring gbm, hence
breaking up the dependency of configurations.

tests/cpp/tree/test_refresh.cc

@@ -1,8 +1,9 @@
 /**
- * Copyright 2018-2013 by XGBoost Contributors
+ * Copyright 2018-2023 by XGBoost Contributors
  */
 #include <gtest/gtest.h>
 #include <xgboost/host_device_vector.h>
+#include <xgboost/task.h>  // for ObjInfo
 #include <xgboost/tree_updater.h>
 
 #include <memory>
@@ -12,9 +13,7 @@
 #include "../../../src/tree/param.h"  // for TrainParam
 #include "../helpers.h"
 
-namespace xgboost {
-namespace tree {
-
+namespace xgboost::tree {
 TEST(Updater, Refresh) {
   bst_row_t constexpr kRows = 8;
   bst_feature_t constexpr kCols = 16;
@@ -33,8 +32,9 @@ TEST(Updater, Refresh) {
   auto ctx = CreateEmptyGenericParam(GPUIDX);
   tree.param.UpdateAllowUnknown(cfg);
   std::vector<RegTree*> trees{&tree};
-  std::unique_ptr<TreeUpdater> refresher(
-      TreeUpdater::Create("refresh", &ctx, ObjInfo{ObjInfo::kRegression}));
+
+  ObjInfo task{ObjInfo::kRegression};
+  std::unique_ptr<TreeUpdater> refresher(TreeUpdater::Create("refresh", &ctx, &task));
 
   tree.ExpandNode(0, 2, 0.2f, false, 0.0, 0.2f, 0.8f, 0.0f, 0.0f,
                   /*left_sum=*/0.0f, /*right_sum=*/0.0f);
@@ -57,6 +57,4 @@ TEST(Updater, Refresh) {
   ASSERT_NEAR(0, tree.Stat(1).loss_chg, kEps);
   ASSERT_NEAR(0, tree.Stat(2).loss_chg, kEps);
 }
-
-}  // namespace tree
-}  // namespace xgboost
+}  // namespace xgboost::tree