Implement extend method for meta info. (#5800)

* Implement extend for host device vector.

include/xgboost/data.h

@@ -158,6 +158,16 @@ class MetaInfo {
    */
   void SetInfo(const char* key, std::string const& interface_str);
 
+  /*
+   * \brief Extend with other MetaInfo.
+   *
+   * \param that The other MetaInfo object.
+   *
+   * \param accumulate_rows Whether rows need to be accumulated in this function.  If
+   *        client code knows number of rows in advance, set this parameter to false.
+   */
+  void Extend(MetaInfo const& that, bool accumulate_rows);
+
  private:
   /*! \brief argsort of labels */
   mutable std::vector<size_t> label_order_cache_;

include/xgboost/host_device_vector.h

@@ -51,6 +51,7 @@
 
 #include <initializer_list>
 #include <vector>
+#include <type_traits>
 
 #include "span.h"
 
@@ -83,6 +84,8 @@ enum GPUAccess {
 
 template <typename T>
 class HostDeviceVector {
+  static_assert(std::is_standard_layout<T>::value, "HostDeviceVector admits only POD types");
+
  public:
   explicit HostDeviceVector(size_t size = 0, T v = T(), int device = -1);
   HostDeviceVector(std::initializer_list<T> init, int device = -1);
@@ -117,6 +120,8 @@ class HostDeviceVector {
   void Copy(const std::vector<T>& other);
   void Copy(std::initializer_list<T> other);
 
+  void Extend(const HostDeviceVector<T>& other);
+
   std::vector<T>& HostVector();
   const std::vector<T>& ConstHostVector() const;
   const std::vector<T>& HostVector() const {return ConstHostVector(); }