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, add sequential mode: the last write bucket is handled inline by the main process rather than in a child process.
• Add explicit reference cleanup (del) for async request and tensors to aid teardown with PersistentAsyncCaller.
• Emit a validation warning when using async save with dist_ckpt_workers > 1.

What does this PR do ?

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[dimapihtar]

LGTM. Thank you!

[sbak5]

@NVIDIA/core-adlr @NVIDIA/core-nemo
This MR should be merged earlier than others.

[sbak5]

/ok to test 4b5fe6c

[sbak5]

/ok to test 20c9b26

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/22372803140