vLLM's symm_buffer stores topk_ids as GLOBAL expert IDs (0..383).
Our weight tensors are indexed by LOCAL IDs (0..47 per rank).
Each rank r handles experts [r*48, r*48+47]. Without conversion,
topk_ids like 137, 222, 378 would index way out of bounds in the
weight tensor (shape (48, N, K)), producing garbage.
Derive experts_start_idx from the topk_ids and subtract to get
local IDs. This was why all ranks except rank 0 produced zero
expert matches → zero output → garbage text.
biondizzle
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