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_prune.cc

@@ -39,8 +39,8 @@ TEST(Updater, Prune) {
   TrainParam param;
   param.UpdateAllowUnknown(cfg);
 
-  std::unique_ptr<TreeUpdater> pruner(
-      TreeUpdater::Create("prune", &ctx, ObjInfo{ObjInfo::kRegression}));
+  ObjInfo task{ObjInfo::kRegression};
+  std::unique_ptr<TreeUpdater> pruner(TreeUpdater::Create("prune", &ctx, &task));
 
   // loss_chg < min_split_loss;
   std::vector<HostDeviceVector<bst_node_t>> position(trees.size());