use fbgemm gconv in dnnlowp (#16020)

use fbgemm gconv in dnnlowp (#16020)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16020

Needs to go over more iterations. For conv, I think we need a high level interface that abstracts out low-level details of which code path will be taken (acc16, outlier-aware, depth-wise, group conv, ...) otherwise the client code will be complex as can be seen from DNNLOWP Conv ops. This will also help us to make interface more stable.

Reviewed By: dskhudia, jianyuh

Differential Revision: D13588996

fbshipit-source-id: 9afce9e441bcaf20437fcc2874fb9d4165a46bcb