More tests for column split and vertical federated learning (#8985)

Added some more tests for the learner and fit_stump, for both column-wise distributed learning and vertical federated learning.

Also moved the `IsRowSplit` and `IsColumnSplit` methods from the `DMatrix` to the `MetaInfo` since in some places we only have access to the `MetaInfo`. Added a new convenience method `IsVerticalFederatedLearning`.

Some refactoring of the testing fixtures.

tests/cpp/tree/test_fit_stump.cc

@@ -6,11 +6,12 @@
 
 #include "../../src/common/linalg_op.h"
 #include "../../src/tree/fit_stump.h"
+#include "../helpers.h"
 
 namespace xgboost {
 namespace tree {
 namespace {
-void TestFitStump(Context const *ctx) {
+void TestFitStump(Context const *ctx, DataSplitMode split = DataSplitMode::kRow) {
   std::size_t constexpr kRows = 16, kTargets = 2;
   HostDeviceVector<GradientPair> gpair;
   auto &h_gpair = gpair.HostVector();
@@ -22,6 +23,7 @@ void TestFitStump(Context const *ctx) {
   }
   linalg::Vector<float> out;
   MetaInfo info;
+  info.data_split_mode = split;
   FitStump(ctx, info, gpair, kTargets, &out);
   auto h_out = out.HostView();
   for (auto it = linalg::cbegin(h_out); it != linalg::cend(h_out); ++it) {
@@ -45,5 +47,12 @@ TEST(InitEstimation, GPUFitStump) {
   TestFitStump(&ctx);
 }
 #endif  // defined(XGBOOST_USE_CUDA)
+
+TEST(InitEstimation, FitStumpColumnSplit) {
+  Context ctx;
+  auto constexpr kWorldSize{3};
+  RunWithInMemoryCommunicator(kWorldSize, &TestFitStump, &ctx, DataSplitMode::kCol);
+}
+
 }  // namespace tree
 }  // namespace xgboost