Update PyTorch pinned commit semi-automatically (#1597)

Summary:
This setups a nightly job to create a PR to update the pinned PyTorch commit in `.ci/docker/ci_commit_pins/pytorch.txt` and `third-party/pytorch`.  This is still only a semi-automatic process because ET is not yet GHFirst and someone needs to import the PR into fbcode to land it.  Atm, we don't have any automated process to do this or if we even want to implement it because the issue will be resolved once ET become GHFirst.  So, I plan to get this out first before trying to figuring out if there is a work around here.

Note that nightly build of ET will come in a separate PR, which will update how the nightly version on https://github.com/pytorch/executorch/blob/main/install_requirements.sh#L20 is updated.

### Testing

https://github.com/pytorch/executorch/pull/1601.

The test PR was under my name because I was using my GITHUB_TOKEN there.  Once I use the correct `UPDATEBOT_TOKEN` and `GH_PYTORCHBOT_TOKEN`, they will be used instead like what PyTorch does, i.e. https://github.com/pytorch/pytorch/pull/117003

Pull Request resolved: https://github.com/pytorch/executorch/pull/1597

Reviewed By: larryliu0820

Differential Revision: D52790880

Pulled By: huydhn

fbshipit-source-id: 2982d2a644a8404704bf9465e37f8752cc5685b7