[FSDP][3/N] Minor fixes (rename, assert message) (#97663)
This is an easy PR.
- It renames `_shard_indices` to `_shard_param_indices` for consistency.
- It fixes an old mention of `comm_module` in an assert message.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97663
Approved by: https://github.com/rohan-varma
