[FSDP] Clean up `_lazy_init()` (#80185)
This PR cleans up `_lazy_init()`. The explanations are left as PR comments.
Differential Revision: [D37726059](https://our.internmc.facebook.com/intern/diff/D37726059)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80185
Approved by: https://github.com/rohan-varma, https://github.com/zhaojuanmao
