Offline evaluation for slate recommendation

[econti]

Implement OPE method for slate recommendation problems:

https://arxiv.org/pdf/2007.12986.pdf
https://papers.nips.cc/paper/2017/file/5352696a9ca3397beb79f116f3a33991-Paper.pdf
http://pchandar.github.io/static/Gruson2019-a2c9a8576182dcb33019d20a7c7a51b7.pdf