fixing invalid upstream for rebase to viable/strict (#78371)
Testing rebase functionality [#78360](https://github.com/pytorch/pytorch/pull/78360) failed due to `invalid upstream 'viable/strict'`. Fixing by changing branch string to "refs/remotes/origin/viable/strict"
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78371
Approved by: https://github.com/janeyx99
