x/imagegen: add FP4 quantization support for image generation models (#13773)

Add --quantize fp4 support to ollama create for image generation models
(flux2, z-image-turbo), using MLX's affine 4-bit quantization.

Changes:
- Add fp4 to validation in CreateImageGenModel
- Add FP4 case to quantizeTensor (group_size=32, bits=4, affine mode)
- Add GetQuantization() to WeightSource interface for dynamic params
- Update LoadLinearLayer to use quantization params from model metadata

x/create/client/quantize.go

@@ -54,6 +54,9 @@ func quantizeTensor(r io.Reader, name, dtype string, shape []int32, quantize str
 	// Quantize based on quantization type
 	var qweight, scales, qbiases *mlx.Array
 	switch quantize {
+	case "fp4":
+		// affine mode: group_size=32, bits=4
+		qweight, scales, qbiases = mlx.Quantize(arr, 32, 4, "affine")
 	case "fp8":
 		// affine mode: group_size=32, bits=8
 		qweight, scales, qbiases = mlx.Quantize(arr, 32, 8, "affine")

x/create/imagegen.go

@@ -20,10 +20,10 @@ import (
 func CreateImageGenModel(modelName, modelDir, quantize string, createLayer LayerCreator, createTensorLayer QuantizingTensorLayerCreator, writeManifest ManifestWriter, fn func(status string)) error {
 	// Validate quantization type
 	switch quantize {
-	case "", "fp8":
+	case "", "fp4", "fp8":
 		// valid
 	default:
-		return fmt.Errorf("unsupported quantization type %q: supported types are fp8", quantize)
+		return fmt.Errorf("unsupported quantization type %q: supported types are fp4, fp8", quantize)
 	}
 
 	var layers []LayerInfo

x/imagegen/safetensors/loader.go

@@ -17,6 +17,18 @@ type WeightSource interface {
 	GetTensor(name string) (*mlx.Array, error)
 	ListTensors() []string
 	HasTensor(name string) bool
+	Quantization() string // Returns "FP4", "FP8", or ""
+}
+
+// quantizationParams returns groupSize, bits, mode for a quantization type.
+// Returns defaults (32, 8, "affine") for unknown types (backward compatibility).
+func quantizationParams(quantization string) (groupSize, bits int, mode string) {
+	switch strings.ToUpper(quantization) {
+	case "FP4":
+		return 32, 4, "affine"
+	default:
+		return 32, 8, "affine" // FP8 or unknown
+	}
 }
 
 // Transformer allows structs to transform weight arrays before assignment.
@@ -233,19 +245,21 @@ func LoadLinearLayer(weights WeightSource, path string) (nn.LinearLayer, error)
 			qbiases, _ = weights.GetTensor(qbiasPath)
 		}
 
+		groupSize, bits, mode := quantizationParams(weights.Quantization())
+
 		if mlx.MetalIsAvailable() {
 			return &nn.QuantizedLinear{
 				Weight:    weight,
 				Scales:    scales,
 				QBiases:   qbiases,
 				Bias:      bias,
-				GroupSize: 32,
-				Bits:      8,
-				Mode:      "affine",
+				GroupSize: groupSize,
+				Bits:      bits,
+				Mode:      mode,
 			}, nil
 		}
 
-		dequantized := mlx.Dequantize(weight, scales, qbiases, 32, 8, "affine")
+		dequantized := mlx.Dequantize(weight, scales, qbiases, groupSize, bits, mode)
 		return nn.NewLinear(dequantized, bias), nil
 	}
 

x/imagegen/safetensors/safetensors.go

@@ -298,6 +298,11 @@ func (mw *ModelWeights) HasTensor(name string) bool {
 	return ok
 }
 
+// Quantization returns empty string for directory-based weights (not quantized).
+func (mw *ModelWeights) Quantization() string {
+	return ""
+}
+
 // ReleaseAll releases all cached native file handles.
 func (mw *ModelWeights) ReleaseAll() {
 	for path, native := range mw.nativeCache {

x/imagegen/weights.go

@@ -106,6 +106,21 @@ func (mw *ManifestWeights) HasTensor(name string) bool {
 	return ok
 }
 
+// Quantization returns the model's quantization type from model_index.json.
+// Returns empty string if not quantized or unknown.
+func (mw *ManifestWeights) Quantization() string {
+	if mw.manifest == nil {
+		return ""
+	}
+	var index struct {
+		Quantization string `json:"quantization"`
+	}
+	if err := mw.manifest.ReadConfigJSON("model_index.json", &index); err != nil {
+		return ""
+	}
+	return index.Quantization
+}
+
 // ReleaseAll frees all native handles and clears the tensor cache.
 func (mw *ManifestWeights) ReleaseAll() {
 	for _, sf := range mw.nativeCache {