Add the new unstable workflow (#92106)
It's empty at the moment, but would tentatively include ROCm trunk jobs. This adopts the same practice we have for inductor where it's run for every commit on trunk, and on PR with `ciflow/unstable` label
- [x] Allow `ciflow/unstable` as a valid tag https://github.com/pytorch/test-infra/pull/1394
- [x] Create the unstable workflow on PyTorch https://github.com/pytorch/pytorch/pull/92106
- [ ] Gather reliability metrics of ROCm runner
- [ ] Decide if we want to move ROCMs trunk jobs to the unstable workflow
- [ ] Add redness metrics for the unstable workflow
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92106
Approved by: https://github.com/ZainRizvi
