[CUDA] ternary with select op (#2283)

* cuda ternary with select op

* comment + fix

* fix

mlx/backend/cuda/device/ternary_ops.cuh

@@ -0,0 +1,12 @@
+// Copyright © 2025 Apple Inc.
+
+namespace mlx::core::cu {
+
+struct Select {
+  template <typename T>
+  __device__ T operator()(bool condition, T x, T y) {
+    return condition ? x : y;
+  }
+};
+
+} // namespace mlx::core::cu