[grpo] fix async generate

[hjh0119]

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[gemini-code-assist]

Summary of Changes

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This pull request addresses a critical bug in the GRPO trainer related to asynchronous generation in a distributed setting. It refines how output data is managed and merged across different processes, particularly ensuring that non-main processes correctly handle their output states without causing errors, thereby stabilizing the asynchronous generation workflow.

Highlights

• Asynchronous Generation Output Handling: Corrected the assignment of 'outputs' to 'all_outputs' for non-main processes within the '_server_rollout' function, ensuring proper data flow in distributed environments.
• Distributed Output Merging Logic: Introduced a conditional check in 'merge_output_input_data' to immediately return empty outputs for non-main processes when 'async_generate' is enabled, preventing assertion failures and correctly handling distributed asynchronous generation.

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[gemini-code-assist]

Code Review

This pull request introduces two important fixes for the asynchronous generation feature in the GRPO trainer. The first change corrects a variable name in _server_rollout, preventing a NameError. The second change adds a necessary check in _postprocess_rollout_outputs to handle cases where non-main processes receive empty outputs during distributed asynchronous generation, thus avoiding an assertion failure. Both changes are correct and improve the robustness of the async generation functionality.