fix goss with constant hessian (#3077)

src/boosting/gbdt.cpp

@@ -70,12 +70,11 @@ void GBDT::Init(const Config* config, const Dataset* train_data, const Objective
   objective_function_ = objective_function;
   num_tree_per_iteration_ = num_class_;
   if (objective_function_ != nullptr) {
-    is_constant_hessian_ = objective_function_->IsConstantHessian();
     num_tree_per_iteration_ = objective_function_->NumModelPerIteration();
-  } else {
-    is_constant_hessian_ = false;
   }
 
+  is_constant_hessian_ = GetIsConstHessian(objective_function);
+
   tree_learner_ = std::unique_ptr<TreeLearner>(TreeLearner::CreateTreeLearner(config_->tree_learner, config_->device_type, config_.get()));
 
   // init tree learner
@@ -653,11 +652,9 @@ void GBDT::ResetTrainingData(const Dataset* train_data, const ObjectiveFunction*
 
   objective_function_ = objective_function;
   if (objective_function_ != nullptr) {
-    is_constant_hessian_ = objective_function_->IsConstantHessian();
     CHECK_EQ(num_tree_per_iteration_, objective_function_->NumModelPerIteration());
-  } else {
-    is_constant_hessian_ = false;
   }
+  is_constant_hessian_ = GetIsConstHessian(objective_function);
 
   // push training metrics
   training_metrics_.clear();

src/boosting/gbdt.h

@@ -375,6 +375,13 @@ class GBDT : public GBDTBase {
   const char* SubModelName() const override { return "tree"; }
 
  protected:
+  virtual bool GetIsConstHessian(const ObjectiveFunction* objective_function) {
+    if (objective_function != nullptr) {
+      return objective_function->IsConstantHessian();
+    } else {
+      return false;
+    }
+  }
   /*!
   * \brief Print eval result and check early stopping
   */

src/boosting/goss.hpp

@@ -153,6 +153,11 @@ class GOSS: public GBDT {
                                     bag_data_cnt_);
     }
   }
+
+ protected:
+  bool GetIsConstHessian(const ObjectiveFunction*) override {
+    return false;
+  }
 };
 
 }  // namespace LightGBM