[checkpoint] Adopt Planner interface across the board. (#83781)
Change StorageReader and StorageWriter to follow the new SavePlanner / LoadPlanner design.
Add optional planner param to load_state_dict and save_state_dict and implement the new protocol.
This includes a small rework of FileSystem layer to support single file per rank and making fsync optional to match torch.save behavior.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83781
Approved by: https://github.com/wanchaol, https://github.com/fduwjj
