Use GitHub REST API to get the merge base commit SHA (#105098)
Fixes https://github.com/pytorch/pytorch/issues/104713
### Testing
Manual testing locally using #104121 and confirm that the correct merge base commit is returned [803c14490b189f9b755ecb9f2a969876088ea243](https://github.com/pytorch/pytorch/commits/1cb87771c1efef32df7009d75bed08249df8ecad) instead of the wrong value provided by `baseRefOid` (de7b6e55eb0f87f8d822f69bad6b4189a857b460). Here is the JSON output of the GraphQL query for PR info https://paste.sh/TJ-QQWz4#fvE3Y6qoJ8vDkRBZ3vowkZ3m
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105098
Approved by: https://github.com/malfet
