Use DataSplitMode to configure data loading (#8434)

* Use `DataSplitMode` to configure data loading

src/learner.cc

@@ -53,15 +53,6 @@ namespace {
 const char* kMaxDeltaStepDefaultValue = "0.7";
 }  // anonymous namespace
 
-namespace xgboost {
-
-enum class DataSplitMode : int {
-  kAuto = 0, kCol = 1, kRow = 2
-};
-}  // namespace xgboost
-
-DECLARE_FIELD_ENUM_CLASS(xgboost::DataSplitMode);
-
 namespace xgboost {
 Learner::~Learner() = default;
 namespace {
@@ -298,6 +289,7 @@ struct LearnerTrainParam : public XGBoostParameter<LearnerTrainParam> {
         .add_enum("auto", DataSplitMode::kAuto)
         .add_enum("col", DataSplitMode::kCol)
         .add_enum("row", DataSplitMode::kRow)
+        .add_enum("none", DataSplitMode::kNone)
         .describe("Data split mode for distributed training.");
     DMLC_DECLARE_FIELD(disable_default_eval_metric)
         .set_default(false)