[CPP] disable early stopping in dart (#1894)

* [CPP] disable early stopping in dart

* fix compile error

src/boosting/dart.hpp

@@ -79,6 +79,11 @@ public:
     return train_score_updater_->score();
   }
   
+  bool EvalAndCheckEarlyStopping() override {
+    GBDT::OutputMetric(iter_);
+    return false;
+  }
+
 private:
   /*!
   * \brief drop trees based on drop_rate

src/boosting/gbdt.h

@@ -360,7 +360,7 @@ protected:
   /*!
   * \brief Print eval result and check early stopping
   */
-  bool EvalAndCheckEarlyStopping();
+  virtual bool EvalAndCheckEarlyStopping();
 
   /*!
   * \brief reset config for bagging