Define core multi-target regression tree structure. (#8884)

- Define a new tree struct embedded in the `RegTree`.
- Provide dispatching functions in `RegTree`.
- Fix some c++-17 warnings about the use of nodiscard (currently we disable the warning on
  the CI).
- Use uint32_t instead of size_t for `bst_target_t` as it has a defined size and can be used
  as part of dmlc parameter.
- Hide the `Segment` struct inside the categorical split matrix.

src/predictor/gpu_predictor.cu

@@ -1,5 +1,5 @@
-/*!
- * Copyright 2017-2021 by Contributors
+/**
+ * Copyright 2017-2023 by XGBoost Contributors
  */
 #include <GPUTreeShap/gpu_treeshap.h>
 #include <thrust/copy.h>
@@ -25,9 +25,7 @@
 #include "xgboost/tree_model.h"
 #include "xgboost/tree_updater.h"
 
-namespace xgboost {
-namespace predictor {
-
+namespace xgboost::predictor {
 DMLC_REGISTRY_FILE_TAG(gpu_predictor);
 
 struct TreeView {
@@ -35,12 +33,11 @@ struct TreeView {
   common::Span<RegTree::Node const> d_tree;
 
   XGBOOST_DEVICE
-  TreeView(size_t tree_begin, size_t tree_idx,
-           common::Span<const RegTree::Node> d_nodes,
+  TreeView(size_t tree_begin, size_t tree_idx, common::Span<const RegTree::Node> d_nodes,
            common::Span<size_t const> d_tree_segments,
            common::Span<FeatureType const> d_tree_split_types,
            common::Span<uint32_t const> d_cat_tree_segments,
-           common::Span<RegTree::Segment const> d_cat_node_segments,
+           common::Span<RegTree::CategoricalSplitMatrix::Segment const> d_cat_node_segments,
            common::Span<uint32_t const> d_categories) {
     auto begin = d_tree_segments[tree_idx - tree_begin];
     auto n_nodes = d_tree_segments[tree_idx - tree_begin + 1] -
@@ -255,7 +252,7 @@ PredictLeafKernel(Data data, common::Span<const RegTree::Node> d_nodes,
 
                   common::Span<FeatureType const> d_tree_split_types,
                   common::Span<uint32_t const> d_cat_tree_segments,
-                  common::Span<RegTree::Segment const> d_cat_node_segments,
+                  common::Span<RegTree::CategoricalSplitMatrix::Segment const> d_cat_node_segments,
                   common::Span<uint32_t const> d_categories,
 
                   size_t tree_begin, size_t tree_end, size_t num_features,
@@ -290,7 +287,7 @@ PredictKernel(Data data, common::Span<const RegTree::Node> d_nodes,
               common::Span<int const> d_tree_group,
               common::Span<FeatureType const> d_tree_split_types,
               common::Span<uint32_t const> d_cat_tree_segments,
-              common::Span<RegTree::Segment const> d_cat_node_segments,
+              common::Span<RegTree::CategoricalSplitMatrix::Segment const> d_cat_node_segments,
               common::Span<uint32_t const> d_categories, size_t tree_begin,
               size_t tree_end, size_t num_features, size_t num_rows,
               size_t entry_start, bool use_shared, int num_group, float missing) {
@@ -334,7 +331,7 @@ class DeviceModel {
   // Pointer to each tree, segmenting the node array.
   HostDeviceVector<uint32_t> categories_tree_segments;
   // Pointer to each node, segmenting categories array.
-  HostDeviceVector<RegTree::Segment> categories_node_segments;
+  HostDeviceVector<RegTree::CategoricalSplitMatrix::Segment> categories_node_segments;
   HostDeviceVector<uint32_t> categories;
 
   size_t tree_beg_;  // NOLINT
@@ -400,9 +397,9 @@ class DeviceModel {
       h_split_cat_segments.push_back(h_categories.size());
     }
 
-    categories_node_segments =
-        HostDeviceVector<RegTree::Segment>(h_tree_segments.back(), {}, gpu_id);
-    std::vector<RegTree::Segment> &h_categories_node_segments =
+    categories_node_segments = HostDeviceVector<RegTree::CategoricalSplitMatrix::Segment>(
+        h_tree_segments.back(), {}, gpu_id);
+    std::vector<RegTree::CategoricalSplitMatrix::Segment>& h_categories_node_segments =
         categories_node_segments.HostVector();
     for (auto tree_idx = tree_begin; tree_idx < tree_end; ++tree_idx) {
       auto const &src_cats_ptr = model.trees.at(tree_idx)->GetSplitCategoriesPtr();
@@ -542,10 +539,10 @@ void ExtractPaths(
   if (thrust::any_of(dh::tbegin(d_split_types), dh::tend(d_split_types),
                      common::IsCatOp{})) {
     dh::PinnedMemory pinned;
-    auto h_max_cat = pinned.GetSpan<RegTree::Segment>(1);
+    auto h_max_cat = pinned.GetSpan<RegTree::CategoricalSplitMatrix::Segment>(1);
     auto max_elem_it = dh::MakeTransformIterator<size_t>(
         dh::tbegin(d_cat_node_segments),
-        [] __device__(RegTree::Segment seg) { return seg.size; });
+        [] __device__(RegTree::CategoricalSplitMatrix::Segment seg) { return seg.size; });
     size_t max_cat_it =
         thrust::max_element(thrust::device, max_elem_it,
                             max_elem_it + d_cat_node_segments.size()) -
@@ -1028,5 +1025,4 @@ XGBOOST_REGISTER_PREDICTOR(GPUPredictor, "gpu_predictor")
     .describe("Make predictions using GPU.")
     .set_body([](Context const* ctx) { return new GPUPredictor(ctx); });
 
-}  // namespace predictor
-}  // namespace xgboost
+}  // namespace xgboost::predictor