Optimize top-2 expert group score computation in MoE routing. - #5124
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In DeepSeek-style expert group routing, groups of experts are selected based on the sum of their top-2 expert scores. When using `jax.lax.top_k(scores_grouped, k=2)` with small group sizes (such as 32 experts per group in DeepSeek-V3), XLA:TPU falls back to `BitonicSortEmitter` rather than `SortPrefixFusion`, introducing significant sort overhead and register padding. PiperOrigin-RevId: 975986068
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Optimize top-2 expert group score computation in MoE routing.
In DeepSeek-style expert group routing, groups of experts are selected based on the sum of their top-2 expert scores. When using
jax.lax.top_k(scores_grouped, k=2)with small group sizes (such as 32 experts per group in DeepSeek-V3), XLA:TPU falls back toBitonicSortEmitterrather thanSortPrefixFusion, introducing significant sort overhead and register padding.