Unify evaluation functions. (#6037)

src/common/hist_util.h

@@ -659,8 +659,6 @@ class GHistBuilder {
   /*! \brief number of all bins over all features */
   uint32_t nbins_ { 0 };
 };
-
-
 }  // namespace common
 }  // namespace xgboost
 #endif  // XGBOOST_COMMON_HIST_UTIL_H_

src/common/observer.h

@@ -6,6 +6,7 @@
 #define XGBOOST_COMMON_OBSERVER_H_
 
 #include <iostream>
+#include <algorithm>
 #include <limits>
 #include <string>
 #include <vector>
@@ -62,6 +63,13 @@ class TrainingObserver {
     auto const& tree = *p_tree;
     this->Observe(tree);
   }
+  template <typename T>
+  void Observe(common::Span<T> span, std::string name,
+               size_t n = std::numeric_limits<std::size_t>::max()) {
+    std::vector<T> copy(span.size());
+    std::copy(span.cbegin(), span.cend(), copy.begin());
+    this->Observe(copy, name, n);
+  }
   /*\brief Observe data hosted by `std::vector'. */
   template <typename T>
   void Observe(std::vector<T> const& h_vec, std::string name,
@@ -71,7 +79,7 @@ class TrainingObserver {
 
     for (size_t i = 0; i < h_vec.size(); ++i) {
       OBSERVER_PRINT << h_vec[i] << ", ";
-      if (i % 8 == 0) {
+      if (i % 8 == 0 && i != 0) {
         OBSERVER_PRINT << OBSERVER_NEWLINE;
       }
       if ((i + 1) == n) {

src/common/row_set.h

@@ -24,13 +24,13 @@ class RowSetCollection {
   struct Elem {
     const size_t* begin{nullptr};
     const size_t* end{nullptr};
-    int node_id{-1};
+    bst_node_t node_id{-1};
       // id of node associated with this instance set; -1 means uninitialized
     Elem()
          = default;
     Elem(const size_t* begin,
          const size_t* end,
-         int node_id = -1)
+         bst_node_t node_id = -1)
         : begin(begin), end(end), node_id(node_id) {}
 
     inline size_t Size() const {