[shard] make state_dict hook be consistent
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79650
for root module we shouldn't accidentally add a "." for state_dict keys, it
should be empty instead to match the module.state_dict behavior
Differential Revision: [D37191203](https://our.internmc.facebook.com/intern/diff/D37191203/)
**NOTE FOR REVIEWERS**: This PR has internal Facebook specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D37191203/)!
Approved by: https://github.com/pritamdamania87, https://github.com/fduwjj
