Add cherry-pick workflow (#119352)
After https://github.com/pytorch/test-infra/pull/4758, we can create a new workflow on PyTorch to receive `try-cherry-pick` dispatch event from the bot, and create the cherry pick PR.
* [x] Cherry pick a PR after it has been landed and create a cherry pick PR to the target release branch.
* [ ] The second part after this is to update the release tracker with the info. This will be done in a subsequent PR.
* [ ] ghstack is not yet supported
* [ ] Cherry pick a reverted commit is not yet supported (from @kit1980 comment)
### Testing
The script can be used locally:
```
python cherry_pick.py --onto release/2.2 --classification release --github-actor huydhn 118907
The cherry pick PR is at https://github.com/pytorch/pytorch/pull/119351
```
The test cherry pick PR is created at https://github.com/pytorch/pytorch/pull/119351
Unit testing this on CI is tricky, so I test this out on canary instead.
* https://github.com/pytorch/pytorch-canary/pull/193#issuecomment-1933162707 creates the PR at https://github.com/pytorch/pytorch-canary/pull/201
* One more test on canary with the new token https://github.com/pytorch/pytorch-canary/pull/193#issuecomment-1933229483. The minimum required permission from what I see is `workflow`
* Cherry picking conflicts could happen and needs to be handled manually https://github.com/pytorch/pytorch-canary/pull/194#issuecomment-1933142975
* ~Require a linked issue when cherry picking regressions, critical fixes, or fixing new features https://github.com/pytorch/pytorch-canary/pull/193#issuecomment-1933174520~ Relax this requirement to a suggestion
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119352
Approved by: https://github.com/atalman
