[Breaking] Don't save leaf child count in JSON. (#6094)

The field is deprecated and not used anywhere in XGBoost.

demo/json-model/json_parser.py

@@ -18,7 +18,6 @@ class Tree:
     _loss_chg = 0
     _sum_hess = 1
     _base_weight = 2
-    _child_cnt = 3
 
     def __init__(self, tree_id: int, nodes, stats):
         self.tree_id = tree_id
@@ -37,10 +36,6 @@ class Tree:
         '''Base weight of a node.'''
         return self.stats[node_id][self._base_weight]
 
-    def num_children(self, node_id: int):
-        '''Number of children of a node.'''
-        return self.stats[node_id][self._child_cnt]
-
     def split_index(self, node_id: int):
         '''Split feature index of node.'''
         return self.nodes[node_id][self._ind]
@@ -138,7 +133,6 @@ class Model:
             base_weights = tree['base_weights']
             loss_changes = tree['loss_changes']
             sum_hessian = tree['sum_hessian']
-            leaf_child_counts = tree['leaf_child_counts']
 
             stats = []
             nodes = []
@@ -152,7 +146,7 @@ class Model:
                 ])
                 stats.append([
                     loss_changes[node_id], sum_hessian[node_id],
-                    base_weights[node_id], leaf_child_counts[node_id]
+                    base_weights[node_id]
                 ])
 
             tree = Tree(tree_id, nodes, stats)

doc/model.schema

@@ -58,12 +58,6 @@
                       "type": "number"
                     }
                   },
-                  "leaf_child_counts": {
-                    "type": "array",
-                    "items": {
-                      "type": "integer"
-                    }
-                  },
                   "left_children": {
                     "type": "array",
                     "items": {
@@ -106,7 +100,6 @@
                   "loss_changes",
                   "sum_hessian",
                   "base_weights",
-                  "leaf_child_counts",
                   "left_children",
                   "right_children",
                   "parents",

src/tree/tree_model.cc

@@ -783,8 +783,6 @@ void RegTree::LoadModel(Json const& in) {
   CHECK_EQ(sum_hessian.size(), n_nodes);
   auto const& base_weights = get<Array const>(in["base_weights"]);
   CHECK_EQ(base_weights.size(), n_nodes);
-  auto const& leaf_child_counts = get<Array const>(in["leaf_child_counts"]);
-  CHECK_EQ(leaf_child_counts.size(), n_nodes);
   // nodes
   auto const& lefts = get<Array const>(in["left_children"]);
   CHECK_EQ(lefts.size(), n_nodes);
@@ -822,7 +820,6 @@ void RegTree::LoadModel(Json const& in) {
     s.loss_chg = get<Number const>(loss_changes[i]);
     s.sum_hess = get<Number const>(sum_hessian[i]);
     s.base_weight = get<Number const>(base_weights[i]);
-    s.leaf_child_cnt = get<Integer const>(leaf_child_counts[i]);
 
     auto& n = nodes_[i];
     bst_node_t left = get<Integer const>(lefts[i]);
@@ -888,7 +885,6 @@ void RegTree::SaveModel(Json* p_out) const {
   std::vector<Json> loss_changes(n_nodes);
   std::vector<Json> sum_hessian(n_nodes);
   std::vector<Json> base_weights(n_nodes);
-  std::vector<Json> leaf_child_counts(n_nodes);
 
   // nodes
   std::vector<Json> lefts(n_nodes);
@@ -906,7 +902,6 @@ void RegTree::SaveModel(Json* p_out) const {
     loss_changes[i] = s.loss_chg;
     sum_hessian[i] = s.sum_hess;
     base_weights[i] = s.base_weight;
-    leaf_child_counts[i] = static_cast<I>(s.leaf_child_cnt);
 
     auto const& n = nodes_[i];
     lefts[i] = static_cast<I>(n.LeftChild());
@@ -938,7 +933,6 @@ void RegTree::SaveModel(Json* p_out) const {
   out["loss_changes"] = std::move(loss_changes);
   out["sum_hessian"] = std::move(sum_hessian);
   out["base_weights"] = std::move(base_weights);
-  out["leaf_child_counts"] = std::move(leaf_child_counts);
 
   out["left_children"] = std::move(lefts);
   out["right_children"] = std::move(rights);