Add single-process checkpoint save to avoid forked multiprocessing

[sbak5]

• Add --dist-ckpt-workers (default 1) to control async checkpoint parallelism. When set to 1, writes run in the main process (sequential mode) instead of forking workers, avoiding multiprocessing-related instability (e.g. CUDA context, memory pinning).
• In FileSystemWriterAsync, handle the last write bucket inline in the main process rather than in a child worker.
• Create async checkpoint workers in threads instead of forked processes.
• Add explicit reference cleanup (del) for async requests and tensors to aid teardown of PersistentAsyncCaller.

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[sbak5]

Opened again to fix issues with #3424

[dimapihtar]

LGTM. Thank you!

[dimapihtar]

/ok to test a654ba4

[sbak5]

/ok to test 7b22753

[sbak5]

/ok to test 70e73f5

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/22502127635