Adding ScaNN library to do MIPS?
I haven't played with it (yet) but it sounds really cool !

@lhoestq Hi I am thinking of adding this new google library to do the MIPS similar to **add_faiss_idex**. As the paper suggests, it is really fast when it comes to retrieving the nearest neighbors. 

https://github.com/google-research/google-research/tree/master/scann

![image](https://user-images.githubusercontent.com/16892570/112738294-78ec9800-8fc6-11eb-9a5f-3d7ee5818e76.png)
