Modify GBTree config

src/gbm/gbtree.cc

@@ -319,6 +319,10 @@ void GBTree::CommitModel(std::vector<std::vector<std::unique_ptr<RegTree>>>&& ne
 void GBTree::LoadConfig(Json const& in) {
   CHECK_EQ(get<String>(in["name"]), "gbtree");
   fromJson(in["gbtree_train_param"], &tparam_);
+  // Process type cannot be kUpdate from loaded model
+  // This would cause all trees to be pushed to trees_to_update
+  // e.g. updating a model, then saving and loading it would result in an empty model
+  tparam_.process_type = TreeProcessType::kDefault;
   int32_t const n_gpus = xgboost::common::AllVisibleGPUs();
   if (n_gpus == 0 && tparam_.predictor == PredictorType::kGPUPredictor) {
     LOG(WARNING)
@@ -348,6 +352,13 @@ void GBTree::SaveConfig(Json* p_out) const {
   auto& out = *p_out;
   out["name"] = String("gbtree");
   out["gbtree_train_param"] = toJson(tparam_);
+
+  // Process type cannot be kUpdate from loaded model
+  // This would cause all trees to be pushed to trees_to_update
+  // e.g. updating a model, then saving and loading it would result in an empty
+  // model
+  out["gbtree_train_param"]["process_type"] = String("default");
+
   out["updater"] = Object();
 
   auto& j_updaters = out["updater"];

src/learner.cc

@@ -731,12 +731,6 @@ class LearnerIO : public LearnerConfiguration {
       tparam_.dsplit = DataSplitMode::kRow;
     }
 
-    // Do not load "process_type":"update", this causes all trees to be removed
-    // from the model on configuration
-    if (cfg_.find("process_type") == cfg_.end()) {
-      cfg_["process_type"] = "default";
-    }
-
     this->Configure();
   }