Feature: implementing the Cascade-DR estimator for SlateOPE

[aiueola]

new feature

• implement SlateCascadeDoublyRobust (Cascade-DR) estimator.
• implement SlateRegressionModel to derive :math:\\hat{Q}_k.
• modified SlateOffPolicyEvaluation to handle Cascade-DR inputs.
• added calc_evaluation_policy_action_dist function in SyntheticSlateBanditDataset.
• implement check_cascade_dr_inputs function.

reference

Haruka Kiyohara, Yuta Saito, Tatsuya Matsuhiro, Yusuke Narita, Nobuyuki Shimizu, and Yasuo Yamamoto.
"Doubly Robust Off-Policy Evaluation for Ranking Policies under the Cascade Behavior Model.", 2022.

TODO

add the following tests

• test_dr_estimators_slate.py
• test_regression_models_slate.py
• test_meta_slate.py
• test_synthetic_slate.py

[usaito]

@aiueola Can you press "Resolve conversation" when you finish incorporating each? It would help me address your replies a lot

[fullflu]

Added a few imo comments! (LGTM if they are fixed!)