Add functionality for rebasebot to rebase onto viable/strict branch (#78276)
Fixes #325
**Summary**: Currently, the pytorchbot only allows for rebasing to the master branch. These modifications add functionality for rebasing to the 'viable/strict' branch of pytorch/pytorch by adding a flag to the comment.
**Test Plan:** tested manually on personal fork ([#1](https://github.com/swang392/pytorch/pull/1)), and included a test case in test_tryrebase.py that checks if rebasing to viable/strict branch was successful.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78276
Approved by: https://github.com/clee2000, https://github.com/janeyx99
