Initial support for external memory in gpu_predictor (#4284)

include/xgboost/data.h

@@ -284,6 +284,7 @@ class BatchIteratorImpl {
  public:
   virtual ~BatchIteratorImpl() {}
   virtual BatchIteratorImpl* Clone() = 0;
+  virtual SparsePage& operator*() = 0;
   virtual const SparsePage& operator*() const = 0;
   virtual void operator++() = 0;
   virtual bool AtEnd() const = 0;
@@ -307,6 +308,11 @@ class BatchIterator {
     ++(*impl_);
   }
 
+  SparsePage& operator*() {
+    CHECK(impl_ != nullptr);
+    return *(*impl_);
+  }
+
   const SparsePage& operator*() const {
     CHECK(impl_ != nullptr);
     return *(*impl_);