fix: Make ONNX runtime optional with build tag

Author: zeekay

mlx_onnx.go

@@ -1,4 +1,5 @@
-// +build cgo
+//go:build cgo && onnx
+// +build cgo,onnx
 
 package mlx
 

mlx_onnx_stub.go

@@ -0,0 +1,18 @@
+//go:build !onnx
+// +build !onnx
+
+package mlx
+
+// Stub functions when ONNX is not available
+
+func detectONNXBackend() bool {
+	return false
+}
+
+func getONNXVersion() string {
+	return "not available"
+}
+
+func hasONNXSupport() bool {
+	return false
+}