Fix stateful dataloader checkpointing across processes

[jrsmartin]

What does this PR do?

This saves stateful dataloader checkpoints per process during distributed training. Stateful dataloader can hold process-specific cursor, dataset, worker, and RNG state. Currently, every process currently writes its state to the same dl_state_dict.bin filename in a shared checkpoint directory, so the final file therefore contains whichever process wrote last, and other processes can restore the wrong data position.

Distributed checkpoints now use rank-qualified filenames such as dl_state_dict_rank0.bin. Loading prefers the current process's file and falls back to the legacy unqualified filename, preserving compatibility with existing checkpoints. Single-process checkpoint filenames remain unchanged.

Related context

I didn't find an existing issue or forum thread for this exact shared-filename collision but #3080 discusses a separate multi-worker/prefetch issue and a comment there describes manually saving dataloader state on every rank. Please note that this PR doesn't address the multi-worker issue in #3080.

Tests

• Added a two-process regression using distinct per-rank dataset streams and a shared checkpoint directory. It verifies exact batch replay after restore and asserts that both rank-qualified state files exist.
• Added a CPU/Gloo entry point so the distributed regression runs in the normal pull-request test suite as well as the existing multi-device suite. It skips when the optional torchdata dependency is unavailable.
• Added unit coverage for single-process naming, multiple dataloaders, process-specific load precedence, and legacy fallback.
• Verified the regression failed before the implementation and passes afterward.
• make quality
• pytest -q tests/test_state_checkpointing.py -k "not map_location" — 18 passed
• pytest -q tests/test_accelerator.py -k stateful_dataloader — 17 passed
• CPU/Gloo two-process regression — passed
• Two-GPU distributed data-loop suite — passed

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline, Pull Request section?
• Was this discussed/approved via a GitHub issue or the forum? No prior discussion was found for this exact issue, but the issue is discussed in Adding ssh_port support #3038
• Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed.

[jrsmartin]

@SunMarc would you mind reviewing this PR when you have a chance - thank you!