Add XGBoosterGetNumFeature (#5856)

- add GetNumFeature to Learner
- add XGBoosterGetNumFeature to C API
- update c-api-demo accordingly

include/xgboost/c_api.h

@@ -563,6 +563,14 @@ XGB_DLL int XGBoosterSetParam(BoosterHandle handle,
                               const char *name,
                               const char *value);
 
+/*!
+ * \brief get number of features
+ * \param out number of features
+ * \return 0 when success, -1 when failure happens
+ */
+XGB_DLL int XGBoosterGetNumFeature(BoosterHandle handle,
+                                   bst_ulong *out);
+
 /*!
  * \brief update the model in one round using dtrain
  * \param handle handle

include/xgboost/learner.h

@@ -158,6 +158,12 @@ class Learner : public Model, public Configurable, public rabit::Serializable {
    */
   virtual void SetParam(const std::string& key, const std::string& value) = 0;
 
+  /*!
+   * \brief Get the number of features of the booster.
+   * \return number of features
+   */
+  virtual uint32_t GetNumFeature() = 0;
+
   /*!
    * \brief Set additional attribute to the Booster.
    *