Pass pointer to model parameters. (#5101)

* Pass pointer to model parameters.

This PR de-duplicates most of the model parameters except the one in
`tree_model.h`.  One difficulty is `base_score` is a model property but can be
changed at runtime by objective function.  Hence when performing model IO, we
need to save the one provided by users, instead of the one transformed by
objective.  Here we created an immutable version of `LearnerModelParam` that
represents the value of model parameter after configuration.

tests/cpp/helpers.h

@@ -21,6 +21,8 @@
 #include <xgboost/json.h>
 #include <xgboost/predictor.h>
 #include <xgboost/generic_parameters.h>
+#include <xgboost/c_api.h>
+#include <xgboost/learner.h>
 
 #include "../../src/common/common.h"
 #include "../../src/common/hist_util.h"
@@ -204,7 +206,12 @@ std::unique_ptr<DMatrix> CreateSparsePageDMatrixWithRC(
     size_t n_rows, size_t n_cols, size_t page_size, bool deterministic,
     const dmlc::TemporaryDirectory& tempdir = dmlc::TemporaryDirectory());
 
-gbm::GBTreeModel CreateTestModel();
+gbm::GBTreeModel CreateTestModel(LearnerModelParam const* param);
+
+std::unique_ptr<GradientBooster> CreateTrainedGBM(
+    std::string name, Args kwargs, size_t kRows, size_t kCols,
+    LearnerModelParam const* learner_model_param,
+    GenericParameter const* generic_param);
 
 inline GenericParameter CreateEmptyGenericParam(int gpu_id) {
   xgboost::GenericParameter tparam;