Automatically update the functorch pinned commit (#81404)
In the short term we want to give people a mechanism to tell which
commit of functorch works with which commit of PyTorch. This PR adds a
job that updates the pinned commit of functorch daily.
Test Plan:
- code reading
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81404
Approved by: https://github.com/ezyang, https://github.com/malfet
