[wip] Topk weights

[aleozlx]

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[gemini-code-assist]

Summary of Changes

Hello @aleozlx, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the flexibility of the fused Mixture of Experts (MoE) kernel by enabling it to process pre-computed routing decisions. This allows for scenarios where expert selection and weighting are determined externally, rather than solely relying on the kernel's internal routing logits computation. The change facilitates integration with advanced routing strategies and optimizes the MoE execution pipeline by providing direct control over expert assignments.

Highlights

• Pre-computed Routing Support: Introduced a new mode for the fused Mixture of Experts (MoE) kernel to accept pre-computed routing information (expert IDs and weights) directly, bypassing the need to compute routing from raw logits.
• Python API Update: The trtllm_fp4_block_scale_routed_moe Python function now explicitly accepts topk_weights as a separate input, alongside topk_ids, clarifying the input structure for pre-computed routing.
• C++ Kernel Interface Enhancement: The underlying C++ Runner::run function has been updated to include an expertIds parameter, enabling the kernel to utilize pre-determined expert assignments.
• Conditional Routing Logic: Modified the C++ routing execution logic to dynamically choose between computing routing from provided logits or using the newly introduced expertIds for pre-computed routing.

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[gemini-code-assist]

Code Review

The pull request introduces support for pre-computed routing weights and expert IDs in the FP4 block scale MoE kernels. The changes are consistently applied across the C++ and Python code, updating function signatures, call sites, and internal logic to accommodate the new parameters. The accompanying documentation updates in the Python file are clear and helpful. The implementation appears to be functionally sound and well-integrated.