C++ API parity: Unfold (#27809)
Summary:
Adds `unfold` functional and module support for the C++ API.
Issue: https://github.com/pytorch/pytorch/issues/25883
Reviewer: yf225
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27809
Differential Revision: D17901792
Pulled By: yf225
fbshipit-source-id: ff58a1866bf240f37ebc589463c60593b8931f51
