Extensible binary serialization format for DMatrix::MetaInfo (#5187)

* Turn xgboost::DataType into C++11 enum class

* New binary serialization format for DMatrix::MetaInfo

* Fix clang-tidy

* Fix c++ test

* Implement new format proposal

* Move helper functions to anonymous namespace; remove unneeded field

* Fix lint

* Add shape.

* Keep only roundtrip test.

* Fix test.

* various fixes

* Update data.cc

Co-authored-by: Jiaming Yuan <jm.yuan@outlook.com>

include/xgboost/data.h

@@ -26,7 +26,7 @@ namespace xgboost {
 class DMatrix;
 
 /*! \brief data type accepted by xgboost interface */
-enum DataType {
+enum class DataType : uint8_t {
   kFloat32 = 1,
   kDouble = 2,
   kUInt32 = 3,
@@ -38,6 +38,9 @@ enum DataType {
  */
 class MetaInfo {
  public:
+  /*! \brief number of data fields in MetaInfo */
+  static constexpr uint64_t kNumField = 7;
+
   /*! \brief number of rows in the data */
   uint64_t num_row_{0};
   /*! \brief number of columns in the data */

include/xgboost/host_device_vector.h

@@ -127,6 +127,8 @@ class HostDeviceVector {
 
   void Resize(size_t new_size, T v = T());
 
+  using value_type = T;
+
  private:
   HostDeviceVectorImpl<T>* impl_;
 };