Small refactor for the GPU predictor. (#11735)

Author: trivialfis

src/data/ellpack_page.cuh

@@ -159,6 +159,7 @@ struct EllpackAccessorImpl {
     return this->null_value_ & ((Ind() << NullShift()) - Ind());
   }
   [[nodiscard]] XGBOOST_HOST_DEV_INLINE bst_idx_t NumBins() const { return gidx_fvalue_map.size(); }
+  [[nodiscard]] XGBOOST_HOST_DEV_INLINE bst_idx_t NumRows() const { return n_rows; }
   [[nodiscard]] XGBOOST_HOST_DEV_INLINE size_t NumFeatures() const { return min_fvalue.size(); }
 };