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/tree/hist/test_histogram.cc

@@ -419,9 +419,8 @@ void TestHistogramExternalMemory(BatchParam batch_param, bool is_approx) {
 
     auto cut = common::SketchOnDMatrix(m.get(), batch_param.max_bin, common::OmpGetNumThreads(0),
                                        false, hess);
-    GHistIndexMatrix gmat;
-    gmat.Init(concat, {}, cut, batch_param.max_bin, false, std::numeric_limits<double>::quiet_NaN(),
-              common::OmpGetNumThreads(0));
+    GHistIndexMatrix gmat(concat, {}, cut, batch_param.max_bin, false,
+                          std::numeric_limits<double>::quiet_NaN(), common::OmpGetNumThreads(0));
     single_build.BuildHist(0, gmat, &tree, row_set_collection, nodes, {}, h_gpair);
     single_page = single_build.Histogram()[0];
   }

tests/cpp/tree/test_quantile_hist.cc

@@ -34,8 +34,7 @@ TEST(QuantileHist, Partitioner) {
   auto cuts = common::SketchOnDMatrix(Xy.get(), 64, ctx.Threads());
 
   for (auto const& page : Xy->GetBatches<SparsePage>()) {
-    GHistIndexMatrix gmat;
-    gmat.Init(page, {}, cuts, 64, true, 0.5, ctx.Threads());
+    GHistIndexMatrix gmat(page, {}, cuts, 64, true, 0.5, ctx.Threads());
     bst_feature_t const split_ind = 0;
     common::ColumnMatrix column_indices;
     column_indices.Init(page, gmat, 0.5, ctx.Threads());