[FSDP] Implement _param_fqns() to return all parameter FQNs for the FSDP module (#82595)
_param_fqns() returns the (fqn, param_name, module_name) tuple for all the parameters wrapped by the current FSDP module.
Differential Revision: [D38325223](https://our.internmc.facebook.com/intern/diff/D38325223/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82595
Approved by: https://github.com/rohan-varma
