remove merging label when merge is cancelled (#98967)
Adds a script to get rid of the "merging" label when a job is cancelled.
At the moment this can create a race condition is someone cancels a job and starts a new one, though these cases should be pretty rare especially in cases where its from a new merge command.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98967
Approved by: https://github.com/malfet
