Implement _pad_circular in ATen
Closes #44459
This migrates the python implementation of `_pad_circular` to ATen and
removes the old C++ implementation that had diverged from python.
Note that `pad` can't actually use this until the
forward-compatibility period is over.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73410
Approved by: https://github.com/ezyang
