add cv for python

src/learner/evaluation.h

@@ -80,6 +80,9 @@ class EvalSet{
     }
     return result;
   }
+  inline size_t Size(void) const {
+    return evals_.size();
+  }
 
  private:
   std::vector<const IEvaluator*> evals_;

src/learner/learner-inl.hpp

@@ -244,7 +244,9 @@ class BoostLearner {
       obj_->SetParam(cfg_[i].first.c_str(), cfg_[i].second.c_str());
       gbm_->SetParam(cfg_[i].first.c_str(), cfg_[i].second.c_str());
     }
-    evaluator_.AddEval(obj_->DefaultEvalMetric());
+    if (evaluator_.Size() == 0) {
+      evaluator_.AddEval(obj_->DefaultEvalMetric());
+    }
   }
   /*! 
    * \brief get un-transformed prediction