Improve test coverage with predictor configuration. (#9354)

* Improve test coverage with predictor configuration.

- Test with ext memory.
- Test with QDM.
- Test with dart.

src/data/gradient_index_format.cc

@@ -68,6 +68,7 @@ class GHistIndexRawFormat : public SparsePageFormat<GHistIndexMatrix> {
   }
 
   std::size_t Write(GHistIndexMatrix const& page, common::AlignedFileWriteStream* fo) override {
+    CHECK_NE(page.index.Size(), 0) << "Empty page is not supported.";
     std::size_t bytes = 0;
     bytes += WriteHistogramCuts(page.cut, fo);
     // indptr

src/data/gradient_index_page_source.cc

@@ -1,10 +1,9 @@
-/*!
- * Copyright 2021-2022 by XGBoost Contributors
+/**
+ * Copyright 2021-2023, XGBoost Contributors
  */
 #include "gradient_index_page_source.h"
 
-namespace xgboost {
-namespace data {
+namespace xgboost::data {
 void GradientIndexPageSource::Fetch() {
   if (!this->ReadCache()) {
     if (count_ != 0 && !sync_) {
@@ -21,5 +20,4 @@ void GradientIndexPageSource::Fetch() {
     this->WriteCache();
   }
 }
-}  // namespace data
-}  // namespace xgboost
+}  // namespace xgboost::data