[megatron] fix: megatron async save ckpt fix

[Leem-Li]

What does this PR do?

1、Only when saving checkpoints asynchronously, add the operation to save the latest_checkpointed_iteration.txt file in the finalize_save_fn function.
2、Fix the error caused by the premature deletion of the checkpoint (ckpt) when async_save and remove_previous_ckpt_in_save are enabled simultaneously.
3、When both optimizer_offload and async_save are enabled, the CPU usage of the child process that forks to save checkpoints is reduced.

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[gemini-code-assist]

Code Review

This pull request attempts to fix an issue with saving latest_checkpointed_iteration.txt for Megatron checkpoints. However, the change introduces a critical bug where this tracker file is no longer saved for synchronous checkpointing operations, which will break the auto-resume functionality. I've provided a comment with a suggested fix to address this.

[Leem-Li]

@FightingZhen @ji-huazhong review