Fix stateful dataloader checkpoint cache after loading

[MrCapricornLiu]

Loading a stateful dataloader checkpoint updates the underlying loader but leaves the adapter's cached state unchanged. Saving again before reading another batch therefore writes the previous cursor. A fresh adapter can replay data; an adapter that had advanced further can skip data after restoration.

Refresh the cache after load_state_dict succeeds. The regression covers shard, dispatcher and skip loaders with zero or two workers, both fresh and reused adapters, and checks the actual remaining samples after another save/load.

Validation: the 12 new cases fail before the fix, and all 49 data-loader tests pass afterward. An Accelerator.save_state/load_state training check passes on CPU and eight H800 GPUs, matching data order and SGD momentum updates against uninterrupted training. Repository Ruff lint/format and applicable pre-commit checks pass.