1. Fix a bug in the MACE refinement module.
2. Add several explainers for ranking tasks, e.g., ValidityRankingExplainer, PermutationRankingExplainer, MACEExplainer.
3. Add save and load functions for the supported explainers.
4. Add a RL-based approach for the MACE counterfactual explainer, e.g., set `method = "rl"` when creating a MACE explainer.