Replace dense all-expert forward (16 experts × all tokens) with
permute-dispatch: sort token-expert pairs by expert id, pad to
[R, C, ℓ] (C = max tokens per expert), run 3 bmm calls for the
batched SiTU-GLU activation, then scatter-add weighted results back.
Routed expert FLOPs drop from R·N to R·C (C ≈ N·k/R under uniform
routing). SiTU parameter structure unchanged; checkpoint compatible.
Tests: sparse-vs-dense fwd/bwd equivalence, unselected expert zero
grad, last_capacity tracking.
dela