[MT-TREE] Support prediction cache and model slicing. (#8968)

- Fix prediction range.
- Support prediction cache in mt-hist.
- Support model slicing.
- Make the booster a Python iterable by defining `__iter__`.
- Cleanup removed/deprecated parameters.
- A new field in the output model `iteration_indptr` for pointing to the ranges of trees for each iteration.

tests/cpp/test_serialization.cc

@@ -1,7 +1,10 @@
-// Copyright (c) 2019-2022 by Contributors
+/**
+ * Copyright (c) 2019-2023, XGBoost Contributors
+ */
 #include <gtest/gtest.h>
 #include <xgboost/base.h>
 #include <xgboost/data.h>
+#include <xgboost/feature_map.h>  // for FeatureMap
 #include <xgboost/json.h>
 #include <xgboost/learner.h>