Add trymerge workflow (#71488)
Summary:
This one, will react to `repo_dispatch` event sent by PyTorch Probot
when `pytorchbot merge this` command is issued
At the moment, workflow will only attempt to merge PRs which has not
been created from forked repo and that match rules defined in
`.github/merge_rules.json`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71488
Reviewed By: bigfootjon
Differential Revision: D33665142
Pulled By: malfet
fbshipit-source-id: e22daa1892523e62d7b7a941960636a6514cb7d7
(cherry picked from commit 92059bab073e2cd6ca6e9f946ffc2f956e22895c)
