Replaced std::vector-based interfaces with HostDeviceVector-based interfaces. (#3116)

* Replaced std::vector-based interfaces with HostDeviceVector-based interfaces.

- replacement was performed in the learner, boosters, predictors,
  updaters, and objective functions
- only interfaces used in training were replaced;
  interfaces like PredictInstance() still use std::vector
- refactoring necessary for replacement of interfaces was also performed,
  such as using HostDeviceVector in prediction cache

* HostDeviceVector-based interfaces for custom objective function example plugin.

src/tree/updater_sync.cc

@@ -23,7 +23,7 @@ class TreeSyncher: public TreeUpdater {
  public:
   void Init(const std::vector<std::pair<std::string, std::string> >& args) override {}
 
-  void Update(const std::vector<bst_gpair> &gpair,
+  void Update(HostDeviceVector<bst_gpair> *gpair,
               DMatrix* dmat,
               const std::vector<RegTree*> &trees) override {
     if (rabit::GetWorldSize() == 1) return;