Add Multi VAE

[akihironitta]

• paper: https://arxiv.org/abs/1802.05814
• official (tensorflow): https://github.com/dawenl/vae_cf/blob/master/VAE_ML20M_WWW2018.ipynb
• unofficial (pytorch): https://github.com/younggyoseo/vae-cf-pytorch/blob/master/main.py

[yiweny]

Can you put the model into the package and put the script under examples/?
Also, can you just add the model into the existing benchmark script?
Thanks for contributing!

[akihironitta]

Will follow up with restructuring the code as needed.