Pass infomation about objective to tree methods. (#7385)

* Define the `ObjInfo` and pass it down to every tree updater.

tests/cpp/tree/test_gpu_hist.cu

@@ -275,7 +275,8 @@ void TestHistogramIndexImpl() {
   int constexpr kNRows = 1000, kNCols = 10;
 
   // Build 2 matrices and build a histogram maker with that
-  tree::GPUHistMakerSpecialised<GradientPairPrecise> hist_maker, hist_maker_ext;
+  tree::GPUHistMakerSpecialised<GradientPairPrecise> hist_maker{ObjInfo{ObjInfo::kRegression}},
+      hist_maker_ext{ObjInfo{ObjInfo::kRegression}};
   std::unique_ptr<DMatrix> hist_maker_dmat(
     CreateSparsePageDMatrixWithRC(kNRows, kNCols, 0, true));
 
@@ -333,7 +334,7 @@ int32_t TestMinSplitLoss(DMatrix* dmat, float gamma, HostDeviceVector<GradientPa
     {"gamma", std::to_string(gamma)}
   };
 
-  tree::GPUHistMakerSpecialised<GradientPairPrecise> hist_maker;
+  tree::GPUHistMakerSpecialised<GradientPairPrecise> hist_maker{ObjInfo{ObjInfo::kRegression}};
   GenericParameter generic_param(CreateEmptyGenericParam(0));
   hist_maker.Configure(args, &generic_param);
 
@@ -394,7 +395,7 @@ void UpdateTree(HostDeviceVector<GradientPair>* gpair, DMatrix* dmat,
       {"sampling_method", sampling_method},
   };
 
-  tree::GPUHistMakerSpecialised<GradientPairPrecise> hist_maker;
+  tree::GPUHistMakerSpecialised<GradientPairPrecise> hist_maker{ObjInfo{ObjInfo::kRegression}};
   GenericParameter generic_param(CreateEmptyGenericParam(0));
   hist_maker.Configure(args, &generic_param);
 
@@ -539,7 +540,8 @@ TEST(GpuHist, ExternalMemoryWithSampling) {
 
 TEST(GpuHist, ConfigIO) {
   GenericParameter generic_param(CreateEmptyGenericParam(0));
-  std::unique_ptr<TreeUpdater> updater {TreeUpdater::Create("grow_gpu_hist", &generic_param) };
+  std::unique_ptr<TreeUpdater> updater{
+      TreeUpdater::Create("grow_gpu_hist", &generic_param, ObjInfo{ObjInfo::kRegression})};
   updater->Configure(Args{});
 
   Json j_updater { Object() };

tests/cpp/tree/test_histmaker.cc

@@ -34,7 +34,8 @@ TEST(GrowHistMaker, InteractionConstraint) {
     RegTree tree;
     tree.param.num_feature = kCols;
 
-    std::unique_ptr<TreeUpdater> updater { TreeUpdater::Create("grow_histmaker", &param) };
+    std::unique_ptr<TreeUpdater> updater{
+        TreeUpdater::Create("grow_histmaker", &param, ObjInfo{ObjInfo::kRegression})};
     updater->Configure(Args{
         {"interaction_constraints", "[[0, 1]]"},
         {"num_feature", std::to_string(kCols)}});
@@ -51,7 +52,8 @@ TEST(GrowHistMaker, InteractionConstraint) {
     RegTree tree;
     tree.param.num_feature = kCols;
 
-    std::unique_ptr<TreeUpdater> updater { TreeUpdater::Create("grow_histmaker", &param) };
+    std::unique_ptr<TreeUpdater> updater{
+        TreeUpdater::Create("grow_histmaker", &param, ObjInfo{ObjInfo::kRegression})};
     updater->Configure(Args{{"num_feature", std::to_string(kCols)}});
     updater->Update(&gradients, p_dmat.get(), {&tree});
 

tests/cpp/tree/test_prune.cc

@@ -38,7 +38,8 @@ TEST(Updater, Prune) {
   tree.param.UpdateAllowUnknown(cfg);
   std::vector<RegTree*> trees {&tree};
   // prepare pruner
-  std::unique_ptr<TreeUpdater> pruner(TreeUpdater::Create("prune", &lparam));
+  std::unique_ptr<TreeUpdater> pruner(
+      TreeUpdater::Create("prune", &lparam, ObjInfo{ObjInfo::kRegression}));
   pruner->Configure(cfg);
 
   // loss_chg < min_split_loss;

tests/cpp/tree/test_quantile_hist.cc

@@ -28,7 +28,7 @@ class QuantileHistMock : public QuantileHistMaker {
 
     BuilderMock(const TrainParam &param, std::unique_ptr<TreeUpdater> pruner,
                 DMatrix const *fmat)
-        : RealImpl(1, param, std::move(pruner), fmat) {}
+        : RealImpl(1, param, std::move(pruner), fmat, ObjInfo{ObjInfo::kRegression}) {}
 
    public:
     void TestInitData(const GHistIndexMatrix& gmat,
@@ -230,7 +230,7 @@ class QuantileHistMock : public QuantileHistMaker {
   explicit QuantileHistMock(
       const std::vector<std::pair<std::string, std::string> >& args,
       const bool single_precision_histogram = false, bool batch = true) :
-      cfg_{args} {
+      QuantileHistMaker{ObjInfo{ObjInfo::kRegression}}, cfg_{args} {
     QuantileHistMaker::Configure(args);
     dmat_ = RandomDataGenerator(kNRows, kNCols, 0.8).Seed(3).GenerateDMatrix();
     if (single_precision_histogram) {

tests/cpp/tree/test_refresh.cc

@@ -32,7 +32,8 @@ TEST(Updater, Refresh) {
   auto lparam = CreateEmptyGenericParam(GPUIDX);
   tree.param.UpdateAllowUnknown(cfg);
   std::vector<RegTree*> trees {&tree};
-  std::unique_ptr<TreeUpdater> refresher(TreeUpdater::Create("refresh", &lparam));
+  std::unique_ptr<TreeUpdater> refresher(
+      TreeUpdater::Create("refresh", &lparam, ObjInfo{ObjInfo::kRegression}));
 
   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);

tests/cpp/tree/test_tree_stat.cc

@@ -23,7 +23,7 @@ class UpdaterTreeStatTest : public ::testing::Test {
   void RunTest(std::string updater) {
     auto tparam = CreateEmptyGenericParam(0);
     auto up = std::unique_ptr<TreeUpdater>{
-        TreeUpdater::Create(updater, &tparam)};
+        TreeUpdater::Create(updater, &tparam, ObjInfo{ObjInfo::kRegression})};
     up->Configure(Args{});
     RegTree tree;
     tree.param.num_feature = kCols;