Change functorch pin mechanism to test functorch in pytorch/pytorch (#81918)
We've moved functorch into pytorch/pytorch. Previously, there was a pin
mechanism that tested pytorch/pytorch against a specific commit in the
pytorch/functorch repository.
This PR changes it so that PyTorch CI tests the functorch inside
pytorch/pytorch instead of the functorch in pytorch/functorch.
Test Plan:
- wait for the functorch build+test job (there is only one)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81918
Approved by: https://github.com/kit1980
