mlx: fuse sigmoid router head in glm4_moe_lite

DeepSeek-V2-style aux-loss-free routing computes sigmoid(gates) once but
needs it twice: the raw sigmoid output is gathered after top-k, while the
post-bias negation is the argpartition key. Fuse into a single multi-output
Compiled kernel returning both, saving two launches on the routing path
per token. Exposed as a general SigmoidRouter since the same pattern is
shared across DeepSeek-V2 descendants.

Improves glm4.7 generation performance by approximately 1%.

x/mlxrunner/mlx/act.go

@@ -62,3 +62,25 @@ var LogitSoftcap = Compile2(
 	},
 	Shapeless(),
 )
+
+// sigmoidRouterFused traces the DeepSeek-V2 / GLM-MoE aux-loss-free router
+// head. Two outputs are returned so the pre-bias sigmoid (used to gather
+// per-expert scores after top-k) and the post-bias negation (used as the
+// argpartition key for top-k) share a single kernel.
+var sigmoidRouterFused = Compile(
+	"SigmoidRouter",
+	func(in ...*Array) []*Array {
+		gates, bias := in[0], in[1]
+		orig := gates.Sigmoid()
+		neg := orig.Add(bias).Negative()
+		return []*Array{orig, neg}
+	},
+	Shapeless(),
+)
+
+// SigmoidRouter returns (sigmoid(gates), -(sigmoid(gates)+bias)) as a fused
+// kernel — the DeepSeek-V2 / GLM-MoE aux-loss-free router head.
+func SigmoidRouter(gates, bias *Array) (origScores, negScores *Array) {
+	out := sigmoidRouterFused(gates, bias)
+	return out[0], out[1]
+}

x/models/glm4_moe_lite/glm4_moe_lite.go

@@ -161,21 +161,21 @@ type MoEGate struct {
 func (g *MoEGate) Forward(x *mlx.Array, cfg *Config) (*mlx.Array, *mlx.Array) {
 	gates := g.Gate.Forward(x)
 
-	scores := mlx.Sigmoid(gates)
-	origScores := scores
-
+	var origScores, negScores *mlx.Array
 	if g.EScoreCorrectionBias != nil {
-		scores = mlx.Add(scores, g.EScoreCorrectionBias)
+		origScores, negScores = mlx.SigmoidRouter(gates, g.EScoreCorrectionBias)
+	} else {
+		origScores = mlx.Sigmoid(gates)
+		negScores = mlx.Neg(origScores)
 	}
 
 	topK := cfg.NumExpertsPerTok
-	negScores := mlx.Neg(scores)
 	inds := mlx.Argpartition(negScores, int(topK)-1, -1)
 
 	dims := inds.Dims()
 	inds = mlx.SliceStartStop(inds, []int32{0, 0, 0}, []int32{int32(dims[0]), int32(dims[1]), topK})
 
-	scores = mlx.TakeAlongAxis(origScores, inds, -1)
+	scores := mlx.TakeAlongAxis(origScores, inds, -1)
 
 	if topK > 1 && cfg.NormTopKProb {
 		sumScores := mlx.Sum(scores, -1, true)