Support graphviz plot for multi-target tree. (#10093)

include/xgboost/tree_model.h

@@ -1,5 +1,5 @@
 /**
- * Copyright 2014-2023 by Contributors
+ * Copyright 2014-2024, XGBoost Contributors
  * \file tree_model.h
  * \brief model structure for tree
  * \author Tianqi Chen
@@ -688,6 +688,9 @@ class RegTree : public Model {
     }
     return (*this)[nidx].DefaultLeft();
   }
+  [[nodiscard]] bst_node_t DefaultChild(bst_node_t nidx) const {
+    return this->DefaultLeft(nidx) ? this->LeftChild(nidx) : this->RightChild(nidx);
+  }
   [[nodiscard]] bool IsRoot(bst_node_t nidx) const {
     if (IsMultiTarget()) {
       return nidx == kRoot;