Add script to cancel workflows (#59019)
Summary:
This removes our cancel_redundant_workflows job in favor of GitHub's built in [`concurrency`](https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#concurrency) keyword which limits runs of a particularly named group. Since the group names have to be unique per job per PR, it should end up looking something like `filename-job_name-{pr number | sha (for non-PR workflows)}`. There's also a script to check workflows and ensure that it is being properly gated so people don't forget to add the key in the future.
`ruamel.YAML` also didn't like some of the spacing so that is changed but it also makes it more consistent so �
This also has a minor change of renaming the workflow templates from `.in` to `.j2` which is the standard Jinja2 extension that the VSCode extension automatically picks up for syntax highlighting / errors.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59019
Test Plan: pushed a commit `reset` and then immediately another commit `test`: the jobs from `reset` are cancelled: https://github.com/pytorch/pytorch/actions/runs/880099510
Reviewed By: samestep
Differential Revision: D28722419
Pulled By: driazati
fbshipit-source-id: c547a161877a0583be9d7edb29244b086b6bcad1
