Move xla codegen to aten. (#45241)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/45241
Test Plan: Imported from OSS
Reviewed By: soumith
Differential Revision: D23926750
Pulled By: ailzhang
fbshipit-source-id: f768e24a9baeca9f9df069a62d6f8b94a853a1ee
