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

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

include/xgboost/learner.h

@@ -11,15 +11,16 @@
 #include <dmlc/any.h>
 #include <xgboost/base.h>
 #include <xgboost/feature_map.h>
-#include <xgboost/predictor.h>
 #include <xgboost/generic_parameters.h>
 #include <xgboost/host_device_vector.h>
 #include <xgboost/model.h>
+#include <xgboost/predictor.h>
+#include <xgboost/task.h>
 
-#include <utility>
 #include <map>
 #include <memory>
 #include <string>
+#include <utility>
 #include <vector>
 
 namespace xgboost {
@@ -307,11 +308,13 @@ struct LearnerModelParam {
   uint32_t num_feature { 0 };
   /* \brief number of classes, if it is multi-class classification  */
   uint32_t num_output_group { 0 };
+  /* \brief Current task, determined by objective. */
+  ObjInfo task{ObjInfo::kRegression};
 
   LearnerModelParam() = default;
   // As the old `LearnerModelParamLegacy` is still used by binary IO, we keep
   // this one as an immutable copy.
-  LearnerModelParam(LearnerModelParamLegacy const& user_param, float base_margin);
+  LearnerModelParam(LearnerModelParamLegacy const& user_param, float base_margin, ObjInfo t);
   /* \brief Whether this parameter is initialized with LearnerModelParamLegacy. */
   bool Initialized() const { return num_feature != 0; }
 };

include/xgboost/objective.h

@@ -13,6 +13,7 @@
 #include <xgboost/model.h>
 #include <xgboost/generic_parameters.h>
 #include <xgboost/host_device_vector.h>
+#include <xgboost/task.h>
 
 #include <vector>
 #include <utility>
@@ -72,6 +73,11 @@ class ObjFunction : public Configurable {
   virtual bst_float ProbToMargin(bst_float base_score) const {
     return base_score;
   }
+  /*!
+   * \brief Return task of this objective.
+   */
+  virtual struct ObjInfo Task() const = 0;
+
   /*!
    * \brief Create an objective function according to name.
    * \param tparam Generic parameters.

include/xgboost/task.h

@@ -0,0 +1,39 @@
+/*!
+ * Copyright 2021 by XGBoost Contributors
+ */
+#ifndef XGBOOST_TASK_H_
+#define XGBOOST_TASK_H_
+
+#include <cinttypes>
+
+namespace xgboost {
+/*!
+ * \brief A struct returned by objective, which determines task at hand.  The struct is
+ *        not used by any algorithm yet, only for future development like categorical
+ *        split.
+ *
+ * The task field is useful for tree split finding, also for some metrics like auc.
+ * Lastly, knowing whether hessian is constant can allow some optimizations like skipping
+ * the quantile sketching.
+ *
+ * This struct should not be serialized since it can be recovered from objective function,
+ * hence it doesn't need to be stable.
+ */
+struct ObjInfo {
+  // What kind of problem are we trying to solve
+  enum Task : uint8_t {
+    kRegression = 0,
+    kBinary = 1,
+    kClassification = 2,
+    kSurvival = 3,
+    kRanking = 4,
+    kOther = 5,
+  } task;
+  // Does the objective have constant hessian value?
+  bool const_hess{false};
+
+  explicit ObjInfo(Task t) : task{t} {}
+  ObjInfo(Task t, bool khess) : const_hess{khess} {}
+};
+}  // namespace xgboost
+#endif  // XGBOOST_TASK_H_

include/xgboost/tree_updater.h

@@ -11,16 +11,17 @@
 #include <dmlc/registry.h>
 #include <xgboost/base.h>
 #include <xgboost/data.h>
-#include <xgboost/tree_model.h>
 #include <xgboost/generic_parameters.h>
 #include <xgboost/host_device_vector.h>
-#include <xgboost/model.h>
 #include <xgboost/linalg.h>
+#include <xgboost/model.h>
+#include <xgboost/task.h>
+#include <xgboost/tree_model.h>
 
 #include <functional>
-#include <vector>
-#include <utility>
 #include <string>
+#include <utility>
+#include <vector>
 
 namespace xgboost {
 
@@ -83,7 +84,7 @@ class TreeUpdater : public Configurable {
    * \param name Name of the tree updater.
    * \param tparam A global runtime parameter
    */
-  static TreeUpdater* Create(const std::string& name, GenericParameter const* tparam);
+  static TreeUpdater* Create(const std::string& name, GenericParameter const* tparam, ObjInfo task);
 };
 
 /*!
@@ -91,8 +92,7 @@ class TreeUpdater : public Configurable {
  */
 struct TreeUpdaterReg
     : public dmlc::FunctionRegEntryBase<TreeUpdaterReg,
-                                        std::function<TreeUpdater* ()> > {
-};
+                                        std::function<TreeUpdater*(ObjInfo task)> > {};
 
 /*!
  * \brief Macro to register tree updater.