[FSDP][BE] Remove `_module_to_handles`, `HandleConfig`; use term "fqn"; clarify docs (#90840)
This PR
- Removes `_module_to_handles` since it is no longer used. We instead use `_comm_module_to_handles`.
- Removes `HandleConfig` and stores its fields directly as attributes on `FlatParamHandle`.
- Uses the term `fqn`/`fqns` uniformly in `flat_param.py` instead of `prefixed_param_name` / `prefixed_param_names`.
- Clarifies some documentation.
I am including all of these BE items in the same PR to save CI.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90840
Approved by: https://github.com/rohan-varma
