Fix lite trainer unit test submodule registration (#42714)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42714
Change two unit tests for the lite trainer to register two instances/objects of the same submodule type instead of the same submodule object twice.
Test Plan: Imported from OSS
Reviewed By: iseeyuan
Differential Revision: D22990736
Pulled By: ann-ss
fbshipit-source-id: 2bf56b5cc438b5a5fc3db90d3f30c5c431d3ae77
