Add test to make sure submodule hooks fire
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75421
As part of FSDP work, we will be relying on `_register_load_state_dict_pre_hook` to manage some specific logic related to loading state dicts.
This PR adds a test to ensure that _register_load_state_dict_pre_hook can be
used to register hooks on modules that will be used in a nested way, and then
calling load_state_dict on the overall module still calls those hooks
appropriately.
Differential Revision: [D35434726](https://our.internmc.facebook.com/intern/diff/D35434726/)
Approved by: https://github.com/albanD
