Mark job as unstable dynamically (#102426)
Allow CI jobs to be marked as unstable dynamically. This use the same mechanism to disable job but with a different issue title `UNSTABLE JOB_NAME`.
The action will output a `is-unstable` flag to let the CI know if the current job it's running is unstable. This is similar to the way `keep-going` flag is exposed. Once this is merged, I will follow up with another PR to actually use `is-unstable` flag in CI.
### Testing
* https://github.com/pytorch/pytorch/issues/102297
* `is-unstable` set https://github.com/pytorch/pytorch/actions/runs/5114544576/jobs/9194921978#step:9:172
* Windows CPU jobs are named unstable https://github.com/pytorch/pytorch/actions/runs/5114544576/jobs/9195186715
* https://github.com/pytorch/pytorch/issues/102298
* `is-unstable set https://github.com/pytorch/pytorch/actions/runs/5114543738/jobs/9195036258#step:11:139
* Dynamo jobs are named unstable https://github.com/pytorch/pytorch/actions/runs/5114543738/jobs/9195036258
* https://github.com/pytorch/pytorch/issues/102299
* `is-unstable` set https://github.com/pytorch/pytorch/actions/runs/5114544576/jobs/9194922158#step:9:190
* MacOS test jobs are named unstable https://github.com/pytorch/pytorch/actions/runs/5114544576/jobs/9195007882
* https://github.com/pytorch/pytorch/issues/102433
* `is-unstable` set https://github.com/pytorch/pytorch/actions/runs/5114544572/jobs/9198630766#step:13:265
* https://github.com/pytorch/pytorch/issues/102425 (open temporarily during testing)
* Disabling CI jobs still works correctly https://github.com/pytorch/pytorch/actions/runs/5114543738/jobs/9194904007 (backwards_compat)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102426
Approved by: https://github.com/ZainRizvi
