Implement categorical prediction for CPU and GPU predict leaf. (#7001)

* Categorical prediction with CPU predictor and GPU predict leaf.

* Implement categorical prediction for CPU prediction.
* Implement categorical prediction for GPU predict leaf.
* Refactor the prediction functions to have a unified get next node function.

Co-authored-by: Shvets Kirill <kirill.shvets@intel.com>

tests/cpp/predictor/test_predictor.h

@@ -66,6 +66,8 @@ void TestInplacePrediction(dmlc::any x, std::string predictor,
 void TestPredictionWithLesserFeatures(std::string preditor_name);
 
 void TestCategoricalPrediction(std::string name);
+
+void TestCategoricalPredictLeaf(StringView name);
 }  // namespace xgboost
 
 #endif  // XGBOOST_TEST_PREDICTOR_H_