kvcache: Use Cast instead of Copy for flash attention masks

Flash attention kernels require the mask of the KV cache be a F16
rather than an F32. We can use the GGML operation ggml_cast to do
this rather than doing it ourselves, which allows reuse of a
preallocated buffer in the graph rather than allocating a new one
for each batch. This improves token generation performance with
flash attention by 10-30% (with gpt-oss). This also makes performance
with flash attention better than without it, as expected.

kvcache/causal.go

@@ -378,9 +378,7 @@ func (c *Causal) buildMask(ctx ml.Context) ml.Tensor {
 	maskTensor := ctx.Input().FromFloatSlice(mask, length, batchSize)
 
 	if c.config.MaskDType != ml.DTypeF32 {
-		out := ctx.Input().Empty(c.config.MaskDType, maskTensor.Shape()...)
+		maskTensor = maskTensor.Cast(ctx, c.config.MaskDType)
-		ctx.Forward(maskTensor.Copy(ctx, out))
-		maskTensor = out
 	}
 
 	return maskTensor

ml/backend.go

@@ -396,6 +396,7 @@ type Tensor interface {
 
 	Shape() []int
 	DType() DType
+	Cast(ctx Context, dtype DType) Tensor
 
 	Bytes() []byte
 	Floats() []float32

ml/backend/ggml/ggml.go

@@ -843,23 +843,7 @@ func (c *Context) newTensor(dtype ml.DType, shape []int) ml.Tensor {
 		panic("set Input or Layer before creating tensors")
 	}
 
-	var cdtype uint32
+	cdtype := ggmlDType(dtype)
-	switch dtype {
-	case ml.DTypeF32:
-		cdtype = C.GGML_TYPE_F32
-	case ml.DTypeF16:
-		cdtype = C.GGML_TYPE_F16
-	case ml.DTypeQ80:
-		cdtype = C.GGML_TYPE_Q8_0
-	case ml.DTypeQ40:
-		cdtype = C.GGML_TYPE_Q4_0
-	case ml.DTypeI32:
-		cdtype = C.GGML_TYPE_I32
-	case ml.DTypeMXFP4:
-		cdtype = C.GGML_TYPE_MXFP4
-	default:
-		panic("unsupported dtype")
-	}
 
 	if len(shape) < 1 || shape[0] == 0 {
 		var shape C.int64_t = 0
@@ -1056,6 +1040,32 @@ func (t *Tensor) DType() ml.DType {
 	}
 }
 
+func ggmlDType(dtype ml.DType) uint32 {
+	switch dtype {
+	case ml.DTypeF32:
+		return C.GGML_TYPE_F32
+	case ml.DTypeF16:
+		return C.GGML_TYPE_F16
+	case ml.DTypeQ80:
+		return C.GGML_TYPE_Q8_0
+	case ml.DTypeQ40:
+		return C.GGML_TYPE_Q4_0
+	case ml.DTypeI32:
+		return C.GGML_TYPE_I32
+	case ml.DTypeMXFP4:
+		return C.GGML_TYPE_MXFP4
+	default:
+		panic("unsupported dtype")
+	}
+}
+
+func (t *Tensor) Cast(ctx ml.Context, dtype ml.DType) ml.Tensor {
+	return &Tensor{
+		b: t.b,
+		t: C.ggml_cast(ctx.(*Context).ctx, t.t, ggmlDType(dtype)),
+	}
+}
+
 func (t *Tensor) Neg(ctx ml.Context) ml.Tensor {
 	return &Tensor{
 		b: t.b,