SHAP values for feature contributions (#2438)

* SHAP values for feature contributions

* Fix commenting error

* New polynomial time SHAP value estimation algorithm

* Update API to support SHAP values

* Fix merge conflicts with updates in master

* Correct submodule hashes

* Fix variable sized stack allocation

* Make lint happy

* Add docs

* Fix typo

* Adjust tolerances

* Remove unneeded def

* Fixed cpp test setup

* Updated R API and cleaned up

* Fixed test typo

src/predictor/gpu_predictor.cu

@@ -384,9 +384,10 @@ class GPUPredictor : public xgboost::Predictor {
   void PredictContribution(DMatrix* p_fmat,
                            std::vector<bst_float>* out_contribs,
                            const gbm::GBTreeModel& model,
-                           unsigned ntree_limit) override {
+                           unsigned ntree_limit,
+                           bool approximate) override {
     cpu_predictor->PredictContribution(p_fmat, out_contribs, model,
-                                       ntree_limit);
+                                       ntree_limit, approximate);
   }
 
   void Init(const std::vector<std::pair<std::string, std::string>>& cfg,