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

* Use explicit template parameter for specifying page type.

src/common/hist_util.cu

@@ -440,7 +440,7 @@ struct GPUSketcher {
 
     row_stride_ = 0;
     sketch_container_.reset(new SketchContainer(param_, dmat));
-    for (const auto &batch : dmat->GetRowBatches()) {
+    for (const auto &batch : dmat->GetBatches<SparsePage>()) {
       this->SketchBatch(batch, info);
     }