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_prediction_cache.cc

@@ -9,6 +9,7 @@
 
 #include "../../../src/tree/param.h"  // for TrainParam
 #include "../helpers.h"
+#include "xgboost/task.h"             // for ObjInfo
 
 namespace xgboost {
 
@@ -71,8 +72,8 @@ class TestPredictionCache : public ::testing::Test {
         ctx.gpu_id = Context::kCpuId;
       }
 
-      std::unique_ptr<TreeUpdater> updater{
-          TreeUpdater::Create(updater_name, &ctx, ObjInfo{ObjInfo::kRegression})};
+      ObjInfo task{ObjInfo::kRegression};
+      std::unique_ptr<TreeUpdater> updater{TreeUpdater::Create(updater_name, &ctx, &task)};
       RegTree tree;
       std::vector<RegTree *> trees{&tree};
       auto gpair = GenerateRandomGradients(n_samples_);