[FSDP][6/N] Rename param/module name helpers for clarity (#97666)
This is an easy PR. It has some remaining local changes that I had that I felt clarified naming.
- `_param_fqns` -> `_param_name_infos` since it returns a tuple of `fqn, param_name, module_name`, not only `fqn`. (similarly for `_shared_param_fqns` -> `_shared_param_name_infos`)
- nit: `parameter_module_names` -> `param_module_names` for consistency since we almost never fully spell out `parameter`. (similarly for `shared_parameter_module_names` -> `shared_param_module_names`)
- nit: `full_fqn` -> `fqn_from_global_root`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97666
Approved by: https://github.com/rohan-varma
