Prepare gradient index for Quantile DMatrix. (#8103)

* Prepare gradient index for Quantile DMatrix.

- Implement push batch with adapter batch.
- Implement `GetFvalue` for prediction.

tests/cpp/data/test_gradient_index.cc

@@ -4,6 +4,7 @@
 #include <gtest/gtest.h>
 #include <xgboost/data.h>
 
+#include "../../../src/common/column_matrix.h"
 #include "../../../src/data/gradient_index.h"
 #include "../helpers.h"
 
@@ -65,5 +66,46 @@ TEST(GradientIndex, FromCategoricalBasic) {
     ASSERT_EQ(common::AsCat(x[i]), common::AsCat(bin_value));
   }
 }
+
+TEST(GradientIndex, PushBatch) {
+  size_t constexpr kRows = 64, kCols = 4;
+  bst_bin_t max_bins = 64;
+  float st = 0.5;
+
+  auto test = [&](float sparisty) {
+    auto m = RandomDataGenerator{kRows, kCols, sparisty}.GenerateDMatrix(true);
+    auto cuts = common::SketchOnDMatrix(m.get(), max_bins, common::OmpGetNumThreads(0), false, {});
+    common::HistogramCuts copy_cuts = cuts;
+
+    ASSERT_EQ(m->Info().num_row_, kRows);
+    ASSERT_EQ(m->Info().num_col_, kCols);
+    GHistIndexMatrix gmat{m->Info(), std::move(copy_cuts), max_bins};
+
+    for (auto const &page : m->GetBatches<SparsePage>()) {
+      SparsePageAdapterBatch batch{page.GetView()};
+      gmat.PushAdapterBatch(m->Ctx(), 0, 0, batch, std::numeric_limits<float>::quiet_NaN(), {}, st,
+                            m->Info().num_row_);
+      gmat.PushAdapterBatchColumns(m->Ctx(), batch, std::numeric_limits<float>::quiet_NaN(), 0);
+    }
+    for (auto const &page : m->GetBatches<GHistIndexMatrix>(BatchParam{max_bins, st})) {
+      for (size_t i = 0; i < kRows; ++i) {
+        for (size_t j = 0; j < kCols; ++j) {
+          auto v0 = gmat.GetFvalue(i, j, false);
+          auto v1 = page.GetFvalue(i, j, false);
+          if (sparisty == 0.0) {
+            ASSERT_FALSE(std::isnan(v0));
+          }
+          if (!std::isnan(v0)) {
+            ASSERT_EQ(v0, v1);
+          }
+        }
+      }
+    }
+  };
+
+  test(0.0f);
+  test(0.5f);
+  test(0.9f);
+}
 }  // namespace data
 }  // namespace xgboost