Rewrite Index Op (#278)

Summary:
Pull Request resolved: #278

Full rewrite of index op, so that it completely implements numpy's [advanced indexing](https://numpy.org/doc/stable/user/basics.indexing.html#advanced-indexing) functionality.

Reviewed By: kirklandsign

Differential Revision: D49159988

fbshipit-source-id: aea0746428df25e99f7aa91f78132fd97da36871

Author: manuelcandales