Fix loading old model.

src/learner.cc

@@ -694,15 +694,24 @@ class LearnerIO : public LearnerConfiguration {
       warn_old_model = false;
     }
 
-    if (mparam_.major_version >= 1) {
-      learner_model_param_ = LearnerModelParam(mparam_,
-                                               obj_->ProbToMargin(mparam_.base_score));
-    } else {
+    if (mparam_.major_version < 1) {
       // Before 1.0.0, base_score is saved as a transformed value, and there's no version
       // attribute in the saved model.
-      learner_model_param_ = LearnerModelParam(mparam_, mparam_.base_score);
+      std::string multi{"multi:"};
+      if (!std::equal(tparam_.objective.cbegin(), tparam_.objective.cend(),
+                      multi.begin())) {
+        HostDeviceVector<float> t;
+        t.HostVector().resize(1);
+        t.HostVector().at(0) = mparam_.base_score;
+        this->obj_->PredTransform(&t);
+        auto base_score = t.HostVector().at(0);
+        mparam_.base_score = base_score;
+      }
       warn_old_model = true;
     }
+
+    learner_model_param_ =
+        LearnerModelParam(mparam_, obj_->ProbToMargin(mparam_.base_score));
     if (attributes_.find("objective") != attributes_.cend()) {
       auto obj_str = attributes_.at("objective");
       auto j_obj = Json::Load({obj_str.c_str(), obj_str.size()});
@@ -752,6 +761,7 @@ class LearnerIO : public LearnerConfiguration {
     LearnerModelParamLegacy mparam = mparam_;  // make a copy to potentially modify
     std::vector<std::pair<std::string, std::string> > extra_attr;
     mparam.contain_extra_attrs = 1;
+    std::cout << "mparam.base_score:" << mparam.base_score << std::endl;
 
     {
       std::vector<std::string> saved_params;