Add single element tuple output from to_backend/to_glow (#5029)
Summary:
Pull Request resolved: https://github.com/pytorch/glow/pull/5029
Support single element tuples in to_backend
Test Plan: new unit test for to_glow
Reviewed By: andrewmillspaugh
Differential Revision: D24539869
fbshipit-source-id: fb385a7448167b2b948e70f6af081bcf78f338dc
