Add a new UNSTABLE category in trymerge (#102784)
Per title, after https://github.com/pytorch/pytorch/pull/102426 landed, it makes sense to have a new category for UNSTABLE jobs and handle them accordingly in trymerge.
* The simple approach is to check for `unstable` in the check (job) name. I plan to roll this out first and then see if we need to cover the more complicated, but less popular case, of unstable build job. Specifically, an unstable build job has no `unstable` in its name
* An unstable job is ignored by trymerge. This is the same behavior we have atm when a job is moved to unstable. It's completely ignored
* The update to Dr. CI will come later, so that unstable failures would also be hidden like broken trunk or flaky
### Testing
Leverage the broken trunk Windows CPU job atm and mark Windows CPU jobs as unstable https://github.com/pytorch/pytorch/issues/102297
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102784
Approved by: https://github.com/clee2000
