Update flags in nnc lowering (#70306)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70306
USE_XNNPACK is the right one to enable lowering to prepacked xnnpack based ops
Test Plan: CI
Reviewed By: ZolotukhinM, priyaramani
Differential Revision: D33279375
fbshipit-source-id: d19ded5643f487f7b58c54a860ad39c8d484ed05
