[Composable] fully_shard load_optim test (#102692)
Closes https://github.com/pytorch/pytorch/issues/93280 and adds tests
for this.
Differential Revision: [D46343364](https://our.internmc.facebook.com/intern/diff/D46343364/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102692
Approved by: https://github.com/awgu
