Add RecVAE model (#568)

* Added RecVAE

* add recvae example

* update readme

* update README

* fix conflict

* remove local ndcg func

* removed unnecessary things from the source code.

* Add None Seed

* Remove unsued code. Fix bottleneck

* Added docstring for main class

* sort the imports alphabetically

* fix typo

* reorder import s

* CSR: Compressed Sparse Row, CSC: Compressed Sparse Colum

* reorder imports

* Update models.rst

* Update README.md

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Co-authored-by: SEUNGHEE.oh <seunghee.oh@hp.com>
Co-authored-by: Quoc-Tuan Truong <tqtg@users.noreply.github.com>