Refactor DMatrix to return batches of different page types (#4686)

* Use explicit template parameter for specifying page type.

tests/cpp/data/test_data.cc

@@ -63,7 +63,7 @@ TEST(SparsePage, PushCSCAfterTranspose) {
       CreateSparsePageDMatrix(n_entries, 64UL, filename);
   const int ncols = dmat->Info().num_col_;
   SparsePage page; // Consolidated sparse page
-  for (const auto &batch : dmat->GetRowBatches()) {
+  for (const auto &batch : dmat->GetBatches<xgboost::SparsePage>()) {
     // Transpose each batch and push
     SparsePage tmp = batch.GetTranspose(ncols);
     page.PushCSC(tmp);