[nnc] Loweings for flatten, xnnpack prepack op (#68470)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/68470
Test Plan: Imported from OSS
Reviewed By: ngimel
Differential Revision: D32545261
Pulled By: IvanKobzarev
fbshipit-source-id: b2bf5b3260002bcc40a351a9c56d786b16b69287
