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.

ml/backend.go

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