Add the Rank-weighted Average Treatment Effect (RATE) metric

[jeongyoonlee]

Is your feature request related to a problem? Please describe.
This request is to add a new RATE metric for CATE estimators.

Describe the solution you'd like
Yadlowsky et al. (2021) proposed the Rank-weighted Average Treatment Effect (RATE) metric, which is an extension of the Qini score.

Describe alternatives you've considered
N/A

Additional context
N/A

[aman-coder03]

hey @jeongyoonlee i would love to work on this issue! i have done some research on the RATE metric and read through the Yadlowsky et al. (2021) paper, also looked at the existing qini_score / get_qini / plot_qini implementation to keep the API consistent