Fix ciflow/all label generation (#63954)
Summary:
the `ciflow/all` is automatically added but need to be added before we call `gen_root_job_condition`.
- fix the order of adding `ciflow/all`
- refactor all the string into global constants
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63954
Reviewed By: malfet
Differential Revision: D30545596
Pulled By: zhouzhuojie
fbshipit-source-id: 83ab668f0234488afb855a72e3ebd4503f7f1a78
