Add nn::Flatten to C++ Frontend (#28072)
Summary:
Adds torch::nn::Flatten 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/28072
Differential Revision: D18202778
Pulled By: yf225
fbshipit-source-id: 43345dcbdf2f50d75746bf9a0ba293b84df275ab
