Fix reflection on weak modules, copy attributes (#20190)
Summary:
* Constructs a new type at runtime so that `isinstance` checks work for
weak modules assigned to `ScriptModule`s
* Fix some extraneous names in `__constants__`
* Add `in_features` and `out_features` to `nn.Linear` `__constants__`
Fixes #19363
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20190
Pulled By: driazati
Differential Revision: D15302350
fbshipit-source-id: 1d4d21ed44ab9578a4bc2a72396a82e9bbcd387c
