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/tree/hist/histogram.h

@@ -12,7 +12,7 @@
 #include "../../common/hist_util.h"
 #include "../../data/gradient_index.h"
 #include "expand_entry.h"
-#include "xgboost/tree_model.h"
+#include "xgboost/tree_model.h"  // for RegTree
 
 namespace xgboost {
 namespace tree {
@@ -175,8 +175,8 @@ class HistogramBuilder {
       auto this_local = hist_local_worker_[entry.nid];
       common::CopyHist(this_local, this_hist, r.begin(), r.end());
 
-      if (!(*p_tree)[entry.nid].IsRoot()) {
-        const size_t parent_id = (*p_tree)[entry.nid].Parent();
+      if (!p_tree->IsRoot(entry.nid)) {
+        const size_t parent_id = p_tree->Parent(entry.nid);
         const int subtraction_node_id = nodes_for_subtraction_trick[node].nid;
         auto parent_hist = this->hist_local_worker_[parent_id];
         auto sibling_hist = this->hist_[subtraction_node_id];
@@ -213,8 +213,8 @@ class HistogramBuilder {
       // Merging histograms from each thread into once
       this->buffer_.ReduceHist(node, r.begin(), r.end());
 
-      if (!(*p_tree)[entry.nid].IsRoot()) {
-        auto const parent_id = (*p_tree)[entry.nid].Parent();
+      if (!p_tree->IsRoot(entry.nid)) {
+        auto const parent_id = p_tree->Parent(entry.nid);
         auto const subtraction_node_id = nodes_for_subtraction_trick[node].nid;
         auto parent_hist = this->hist_[parent_id];
         auto sibling_hist = this->hist_[subtraction_node_id];
@@ -237,10 +237,10 @@ class HistogramBuilder {
     common::ParallelFor2d(
         space, this->n_threads_, [&](size_t node, common::Range1d r) {
           const auto &entry = nodes[node];
-          if (!((*p_tree)[entry.nid].IsLeftChild())) {
+          if (!(p_tree->IsLeftChild(entry.nid))) {
             auto this_hist = this->hist_[entry.nid];
 
-            if (!(*p_tree)[entry.nid].IsRoot()) {
+            if (!p_tree->IsRoot(entry.nid)) {
               const int subtraction_node_id = subtraction_nodes[node].nid;
               auto parent_hist = hist_[(*p_tree)[entry.nid].Parent()];
               auto sibling_hist = hist_[subtraction_node_id];
@@ -285,7 +285,7 @@ class HistogramBuilder {
     std::sort(merged_node_ids.begin(), merged_node_ids.end());
     int n_left = 0;
     for (auto const &nid : merged_node_ids) {
-      if ((*p_tree)[nid].IsLeftChild()) {
+      if (p_tree->IsLeftChild(nid)) {
         this->hist_.AddHistRow(nid);
         (*starting_index) = std::min(nid, (*starting_index));
         n_left++;
@@ -293,7 +293,7 @@ class HistogramBuilder {
       }
     }
     for (auto const &nid : merged_node_ids) {
-      if (!((*p_tree)[nid].IsLeftChild())) {
+      if (!(p_tree->IsLeftChild(nid))) {
         this->hist_.AddHistRow(nid);
         this->hist_local_worker_.AddHistRow(nid);
       }