Use ellpack for prediction only when sparsepage doesn't exist. (#5504)

tests/cpp/helpers.cu

@@ -0,0 +1,17 @@
+#include "helpers.h"
+#include "../../src/data/device_adapter.cuh"
+#include "../../src/data/device_dmatrix.h"
+
+namespace xgboost {
+std::shared_ptr<DMatrix> RandomDataGenerator::GenerateDeviceDMatrix(bool with_label,
+                                                                    bool float_label,
+                                                                    size_t classes) {
+  std::vector<HostDeviceVector<float>> storage(cols_);
+  std::string arr = this->GenerateColumnarArrayInterface(&storage);
+  auto adapter = data::CudfAdapter(arr);
+  std::shared_ptr<DMatrix> m {
+    new data::DeviceDMatrix{&adapter,
+          std::numeric_limits<float>::quiet_NaN(), 1, 256}};
+  return m;
+}
+}  // namespace xgboost