Fix serialization (#15033)
Summary:
Fixes a bug where (de-)/serializing a hierarchy of submodules where one submodule doesn't have any parameters, but its submodules do, doesn't get properly loaded. This had to do with the fact that the old protobuf format couldn't store empty parameters.
Fixes https://github.com/pytorch/pytorch/issues/14891
soumith ezyang ebetica
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15033
Differential Revision: D13411322
Pulled By: goldsborough
fbshipit-source-id: 2ef73b2aa93fa9e46b1cbe1fd47d9f134d6016d5
