Prediction by indices (subsample < 1) (#6683)

* Another implementation of predicting by indices

* Fixed omp parallel_for variable type

* Removed SparsePageView from Updater

tests/cpp/predictor/test_cpu_predictor.cc

@@ -183,7 +183,7 @@ TEST(CpuPredictor, InplacePredict) {
   }
 }
 
-TEST(CpuPredictor, UpdatePredictionCache) {
+void TestUpdatePredictionCache(bool use_subsampling) {
   size_t constexpr kRows = 64, kCols = 16, kClasses = 4;
   LearnerModelParam mparam;
   mparam.num_feature = kCols;
@@ -198,6 +198,9 @@ TEST(CpuPredictor, UpdatePredictionCache) {
   std::map<std::string, std::string> cfg;
   cfg["tree_method"] = "hist";
   cfg["predictor"]   = "cpu_predictor";
+  if (use_subsampling) {
+    cfg["subsample"] = "0.5";
+  }
   Args args = {cfg.cbegin(), cfg.cend()};
   gbm->Configure(args);
 
@@ -226,6 +229,11 @@ TEST(CpuPredictor, UpdatePredictionCache) {
   }
 }
 
+TEST(CpuPredictor, UpdatePredictionCache) {
+    TestUpdatePredictionCache(false);
+    TestUpdatePredictionCache(true);
+}
+
 TEST(CpuPredictor, LesserFeatures) {
   TestPredictionWithLesserFeatures("cpu_predictor");
 }