Implement fit stump. (#8607)

src/common/stats.cu

@@ -13,7 +13,7 @@
 
 namespace xgboost {
 namespace common {
-namespace cuda {
+namespace cuda_impl {
 float Median(Context const* ctx, linalg::TensorView<float const, 2> t,
              common::OptionalWeights weights) {
   HostDeviceVector<size_t> segments{0, t.Size()};
@@ -42,6 +42,17 @@ float Median(Context const* ctx, linalg::TensorView<float const, 2> t,
   CHECK_EQ(quantile.Size(), 1);
   return quantile.HostVector().front();
 }
-}  // namespace cuda
+
+void Mean(Context const* ctx, linalg::VectorView<float const> v, linalg::VectorView<float> out) {
+  float n = v.Size();
+  auto it = dh::MakeTransformIterator<float>(
+      thrust::make_counting_iterator(0ul), [=] XGBOOST_DEVICE(std::size_t i) { return v(i) / n; });
+  std::size_t bytes;
+  CHECK_EQ(out.Size(), 1);
+  cub::DeviceReduce::Sum(nullptr, bytes, it, out.Values().data(), v.Size());
+  dh::TemporaryArray<char> temp{bytes};
+  cub::DeviceReduce::Sum(temp.data().get(), bytes, it, out.Values().data(), v.Size());
+}
+}  // namespace cuda_impl
 }  // namespace common
 }  // namespace xgboost