Use Span in GPU exact updater. (#4020)

* Use Span in GPU exact updater.

* Add a small test.

tests/cpp/tree/test_gpu_exact.cu

@@ -0,0 +1,48 @@
+#include <gtest/gtest.h>
+#include <xgboost/tree_updater.h>
+
+#include <vector>
+#include <string>
+#include <utility>
+
+#include "../helpers.h"
+
+namespace xgboost {
+namespace tree {
+
+TEST(GPUExact, Update) {
+  using Arg = std::pair<std::string, std::string>;
+  std::vector<Arg> args{
+    {"n_gpus", "1"},
+    {"gpu_id", "0"},
+    {"max_depth", "1"}};
+
+  auto* p_gpuexact_maker = TreeUpdater::Create("grow_gpu");
+  p_gpuexact_maker->Init(args);
+
+  size_t constexpr n_rows = 4;
+  size_t constexpr n_cols = 8;
+  bst_float constexpr sparsity = 0.0f;
+
+  auto dmat = CreateDMatrix(n_rows, n_cols, sparsity, 3);
+  std::vector<GradientPair> h_gpair(n_rows);
+  for (size_t i = 0; i < n_rows; ++i) {
+      h_gpair[i] = GradientPair(i % 2, 1);
+  }
+  HostDeviceVector<GradientPair> gpair (h_gpair);
+  RegTree tree;
+
+  p_gpuexact_maker->Update(&gpair, (*dmat).get(), {&tree});
+  auto const& nodes = tree.GetNodes();
+  ASSERT_EQ(nodes.size(), 3);
+
+  float constexpr kRtEps = 1e-6;
+  ASSERT_NEAR(tree.Stat(0).sum_hess, 4, kRtEps);
+  ASSERT_NEAR(tree.Stat(1).sum_hess, 2, kRtEps);
+  ASSERT_NEAR(tree.Stat(2).sum_hess, 2, kRtEps);
+
+  ASSERT_NEAR(tree.Stat(0).loss_chg, 0.8f, kRtEps);
+}
+
+}  // namespace tree
+}  // namespace xgboost