Make LTC codegen customizable enough for XLA migration (#76180)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76180
Provides string variables to let xla customize the generated code sufficiently enough to facilitate their migration onto LTC.
Some/all of these custom variables are expected to be short-lived for migration and eventually revert to using the original content that points to LTC functionality.
Test Plan: Imported from OSS
Reviewed By: huiguoo
Differential Revision: D35861778
Pulled By: wconstab
fbshipit-source-id: ef7aae55334628e2e7ff0c22e5c86ab95439256d
(cherry picked from commit 971f075e0c21804558f46c685508bd23daa42d4f)
