Properly mangle `nn.Module.__construct` (#23779)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23779
Mangling is two underscores, not one :(. We want this method to be
private so that inheritors who define a `__construct` do not interfere
with Module initialization
Test Plan: Imported from OSS
Differential Revision: D16645156
Pulled By: suo
fbshipit-source-id: b9060cb35bfaa0391ff200b63fb78b1ac15fee39
