add checkpoint recovery feature

[whyXVI]

• Added checkpoint saving functionality to ensure progress is saved during processing.
• Implemented _save_checkpoint method to save current progress after processing each conversation.
• Implemented _load_checkpoint method to check and load checkpoint data on program startup.
• Modified _process method to resume processing from the last saved checkpoint.
• Ensured checkpoint file is deleted upon completion to avoid redundant processing.

This change enhances the reliability and fault tolerance of the program, particularly for long-running tasks and in bad network environments.

Issue #, if available:

Description of changes:

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