Workflow consolidation for GitHub actions
This PR implements the proposal in https://fb.quip.com/RcLZAOb9k0qI,
which calls for using GitHub's "normal" means of composition (composite
actions, reusable workflows) instead of codegen.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74306
Approved by: https://github.com/janeyx99
