Optimize prepare_lens by removing device transfer

[shengzhaotian]

Removed unnecessary device transfer for cu_seqlens in prepare_lens function.

[gemini-code-assist]

Summary of Changes

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This pull request optimizes the prepare_lens utility function by streamlining its data handling. It removes explicit device transfers that were causing unnecessary movement of tensor data to and from the CPU, thereby enhancing performance and reducing computational overhead by allowing operations to remain on the original processing device.

Highlights

• Device Transfer Removal: Eliminated an unnecessary .cpu() call on cu_seqlens and a subsequent .to(device) call on the tmp tensor within the prepare_lens function.
• Performance Optimization: Improved the efficiency of the prepare_lens function by preventing redundant data transfers between the device (e.g., NPU/GPU) and the CPU, keeping computations on the original device.

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[gemini-code-assist]

Code Review

The pull request correctly optimizes the prepare_lens function by removing an unnecessary device transfer, which is a good performance improvement. I have added a comment with suggestions for further code cleanup and simplification that this change makes possible.