docs: separate autosummary for flatten layers (#54663)
Summary:
fixes https://github.com/pytorch/pytorch/issues/46881
https://11815123-65600975-gh.circle-artifacts.com/0/docs/generated/torch.nn.Flatten.html#torch.nn.Flatten
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54663
Reviewed By: ailzhang
Differential Revision: D27328367
Pulled By: zou3519
fbshipit-source-id: de1651a670181db8ea8ab16624c17ba08a88eb5d
