Initial support for external memory in gpu_predictor (#4284)

src/data/simple_dmatrix.cc

@@ -32,6 +32,10 @@ float SimpleDMatrix::GetColDensity(size_t cidx) {
 class SimpleBatchIteratorImpl : public BatchIteratorImpl {
  public:
   explicit SimpleBatchIteratorImpl(SparsePage* page) : page_(page) {}
+  SparsePage& operator*() override {
+    CHECK(page_ != nullptr);
+    return *page_;
+  }
   const SparsePage& operator*() const override {
     CHECK(page_ != nullptr);
     return *page_;

src/data/sparse_page_dmatrix.cc

@@ -29,6 +29,7 @@ class SparseBatchIteratorImpl : public BatchIteratorImpl {
   explicit SparseBatchIteratorImpl(SparsePageSource* source) : source_(source) {
     CHECK(source_ != nullptr);
   }
+  SparsePage& operator*() override { return source_->Value(); }
   const SparsePage& operator*() const override { return source_->Value(); }
   void operator++() override { at_end_ = !source_->Next(); }
   bool AtEnd() const override { return at_end_; }

src/data/sparse_page_source.cc

@@ -104,6 +104,10 @@ void SparsePageSource::BeforeFirst() {
   }
 }
 
+SparsePage& SparsePageSource::Value() {
+  return *page_;
+}
+
 const SparsePage& SparsePageSource::Value() const {
   return *page_;
 }

src/data/sparse_page_source.h

@@ -43,6 +43,7 @@ class SparsePageSource : public DataSource {
   // implement BeforeFirst
   void BeforeFirst() override;
   // implement Value
+  SparsePage& Value();
   const SparsePage& Value() const override;
   /*!
    * \brief Create source by taking data from parser.