Support categorical splits in in TreeExplainer (rapidsai#4473)

Stacked on top of rapidsai#4447. Do not merge until rapidsai#4447 is merged first.

~If you'd like to review before rapidsai#4447 is merged, look at the net diff at hcho3#2.~

- [x] Test XGBoost models with categorical splits
- [x] Test LightGBM models with categorical splits

Authors:
  - Philip Hyunsu Cho (https://github.com/hcho3)

Approvers:
  - Dante Gama Dessavre (https://github.com/dantegd)

URL: rapidsai#4473

cpp/include/cuml/explainer/tree_shap.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, NVIDIA CORPORATION.
+ * Copyright (c) 2021-2022, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -34,7 +34,7 @@ class TreePathInfo {
 };
 
 std::unique_ptr<TreePathInfo> extract_path_info(ModelHandle model);
-void gpu_treeshap(const TreePathInfo* path_info,
+void gpu_treeshap(TreePathInfo* path_info,
                   const float* data,
                   std::size_t n_rows,
                   std::size_t n_cols,