[FakeLowp] Open source more c2 ops (#38878)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38878
We need to Packing op and shape extraction functions to make some of the FakeLowP tests run in OSS.
Test Plan: unittests
Reviewed By: hyuen
Differential Revision: D21682704
fbshipit-source-id: f36321b91acfd738e90543309b82ad87b9e5c156
