fix __len__, __contains__, getitem inherited from interface class derived from nn container (closes #40603) (#40789)
Summary:
Define static script implementation of __len__ and __contains__ on any subclass derived from a type such as ModuleList, Sequential, or ModuleDict. Implement getitem for classes derived from ModuleDict.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40789
Reviewed By: eellison
Differential Revision: D22325159
Pulled By: wconstab
fbshipit-source-id: fc1562c29640fe800e13b5a1dd48e595c2c7239b
