C++ API Parity: `functional::fold` and `Fold::pretty_print` (#28732)
Summary:
Adds `torch::nn::functional::fold` support and updates `Fold::pretty_print` in the C++ API for more thorough Python parity.
Note: Small updates in source files to maintain consistency elsewhere.
Reviewer: yf225
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28732
Differential Revision: D18219955
Pulled By: yf225
fbshipit-source-id: fd2e9be8f17db77c1b1f384c0d2e16cc34858c0c
