[bugfix] fix grpo loss

[hjh0119]

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[gemini-code-assist]

Summary of Changes

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This pull request addresses a bug in the GRPO loss calculation by reordering the initialization of the loss_mask. This ensures that the mask is consistently available for subsequent operations, regardless of whether logps needs to be computed from logits or is directly provided, thereby enhancing the robustness of the loss function.

Highlights

• GRPO Loss Calculation: Corrected the order of loss_mask initialization in the GRPO loss function to ensure it is always defined before use, preventing potential errors when logps is already available.

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Changelog

• src/twinkle/loss/grpo.py
  • Moved the loss_mask initialization to ensure it's always defined.

Activity

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Code Review

This pull request fixes a bug in the GRPOLoss calculation where loss_mask was not defined in all code paths, which would cause a runtime error. The fix correctly moves the loss_mask calculation to ensure it's always initialized. I've suggested a small improvement to reduce code duplication by reusing an existing helper method, which will improve long-term maintainability.