example quantizer and delegate backend

Summary:
An example to show how to add a new backend. We've demoed this and maybe it's easier to put up a generic implementation so it's easier get start on adding quantizer and delegate.

This demo includes
- example quantizer
- example partitioner
- example backend
- examples operators available on the backend
- passes on memory format permutates

Reviewed By: tarun292

Differential Revision: D49120351

fbshipit-source-id: 1118820b365af9b2cb1eec62477a915e6bb90f6d