[FSDP] Deduplicate `_orig_size` and `_unsharded_size` (#79984)
This removes the `_orig_size` attribute that is initialized in `fully_sharded_data_parallel.py` since it represents the same quantity as `_unsharded_size` in `flat_param.py`. Since the quantity is not sharding dependent, we keep its initialization in `FlatParameter.init_metadata()` instead of in `FullyShardedDataParallel._shard_parameters()`.
Differential Revision: [D37726062](https://our.internmc.facebook.com/intern/diff/D37726062)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79984
Approved by: https://github.com/rohan-varma
