Make QuantileDMatrix default to sklearn esitmators. (#8220)

src/data/iterative_dmatrix.cc

@@ -7,6 +7,7 @@
 
 #include "../common/column_matrix.h"
 #include "../common/hist_util.h"
+#include "../tree/param.h"  // FIXME(jiamingy): Find a better way to share this parameter.
 #include "gradient_index.h"
 #include "proxy_dmatrix.h"
 #include "simple_batch_iterator.h"
@@ -14,6 +15,38 @@
 namespace xgboost {
 namespace data {
 
+IterativeDMatrix::IterativeDMatrix(DataIterHandle iter_handle, DMatrixHandle proxy,
+                                   std::shared_ptr<DMatrix> ref, DataIterResetCallback* reset,
+                                   XGDMatrixCallbackNext* next, float missing, int nthread,
+                                   bst_bin_t max_bin)
+    : proxy_{proxy}, reset_{reset}, next_{next} {
+  // fetch the first batch
+  auto iter =
+      DataIterProxy<DataIterResetCallback, XGDMatrixCallbackNext>{iter_handle, reset_, next_};
+  iter.Reset();
+  bool valid = iter.Next();
+  CHECK(valid) << "Iterative DMatrix must have at least 1 batch.";
+
+  auto d = MakeProxy(proxy_)->DeviceIdx();
+
+  StringView msg{"All batch should be on the same device."};
+  if (batch_param_.gpu_id != Context::kCpuId) {
+    CHECK_EQ(d, batch_param_.gpu_id) << msg;
+  }
+
+  batch_param_ = BatchParam{d, max_bin};
+  // hardcoded parameter.
+  batch_param_.sparse_thresh = tree::TrainParam::DftSparseThreshold();
+
+  ctx_.UpdateAllowUnknown(
+      Args{{"nthread", std::to_string(nthread)}, {"gpu_id", std::to_string(d)}});
+  if (ctx_.IsCPU()) {
+    this->InitFromCPU(iter_handle, missing, ref);
+  } else {
+    this->InitFromCUDA(iter_handle, missing, ref);
+  }
+}
+
 void GetCutsFromRef(std::shared_ptr<DMatrix> ref_, bst_feature_t n_features, BatchParam p,
                     common::HistogramCuts* p_cuts) {
   CHECK(ref_);
@@ -199,6 +232,7 @@ void IterativeDMatrix::InitFromCPU(DataIterHandle iter_handle, float missing,
   if (n_batches == 1) {
     this->info_ = std::move(proxy->Info());
     this->info_.num_nonzero_ = nnz;
+    this->info_.num_col_ = n_features;  // proxy might be empty.
     CHECK_EQ(proxy->Info().labels.Size(), 0);
   }
 }
@@ -210,6 +244,10 @@ BatchSet<GHistIndexMatrix> IterativeDMatrix::GetGradientIndex(BatchParam const&
     ghist_ = std::make_shared<GHistIndexMatrix>(&ctx_, Info(), *ellpack_, param);
   }
 
+  if (param.sparse_thresh != tree::TrainParam::DftSparseThreshold()) {
+    LOG(WARNING) << "`sparse_threshold` can not be changed when `QuantileDMatrix` is used instead "
+                    "of `DMatrix`.";
+  }
   auto begin_iter =
       BatchIterator<GHistIndexMatrix>(new SimpleBatchIteratorImpl<GHistIndexMatrix>(ghist_));
   return BatchSet<GHistIndexMatrix>(begin_iter);