Register quantized ops into quantization example (#85)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/85

Enables the ability to `export_to_pte`. Previously quantized models can't run `export_to_pte` because some quantized ops are missing out variants. Recently we added support for custom ops and register them into EXIR by loading shared library.

This diff adds support for registering quantized ops out variants and add it into CI.

Reviewed By: huydhn

Differential Revision: D48541611

fbshipit-source-id: e8740383abb38704a6450de200572400498ce4d5