Implement fit stump. (#8607)

tests/cpp/common/test_stats.cc

@@ -3,8 +3,10 @@
  */
 #include <gtest/gtest.h>
 #include <xgboost/context.h>
+#include <xgboost/linalg.h>  // Tensor,Vector
 
 #include "../../../src/common/stats.h"
+#include "../../../src/common/transform_iterator.h"  // common::MakeIndexTransformIter
 
 namespace xgboost {
 namespace common {
@@ -69,5 +71,35 @@ TEST(Stats, Median) {
   ASSERT_EQ(m, .5f);
 #endif  // defined(XGBOOST_USE_CUDA)
 }
+namespace {
+void TestMean(Context const* ctx) {
+  std::size_t n{128};
+  linalg::Vector<float> data({n}, ctx->gpu_id);
+  auto h_v = data.HostView().Values();
+  std::iota(h_v.begin(), h_v.end(), .0f);
+
+  auto nf = static_cast<float>(n);
+  float mean = nf * (nf - 1) / 2 / n;
+
+  linalg::Vector<float> res{{1}, ctx->gpu_id};
+  Mean(ctx, data, &res);
+  auto h_res = res.HostView();
+  ASSERT_EQ(h_res.Size(), 1);
+  ASSERT_EQ(mean, h_res(0));
+}
+}  // anonymous namespace
+
+TEST(Stats, Mean) {
+  Context ctx;
+  TestMean(&ctx);
+}
+
+#if defined(XGBOOST_USE_CUDA)
+TEST(Stats, GPUMean) {
+  Context ctx;
+  ctx.UpdateAllowUnknown(Args{{"gpu_id", "0"}});
+  TestMean(&ctx);
+}
+#endif  // defined(XGBOOST_USE_CUDA)
 }  // namespace common
 }  // namespace xgboost

tests/cpp/common/test_stats.cu

@@ -7,6 +7,7 @@
 #include <vector>
 
 #include "../../../src/common/stats.cuh"
+#include "../../../src/common/stats.h"
 #include "xgboost/base.h"
 #include "xgboost/context.h"
 #include "xgboost/host_device_vector.h"