Select the system's default GPU device on which to run your Metal code.
To use the Metal framework, you start by getting a GPU device. All of the objects your app needs to interact with Metal come from a MTLDevice that you acquire at runtime. iOS and tvOS devices have only one GPU that you access by calling MTLCreateSystemDefaultDevice():
On macOS devices that are built with multiple GPUs, like Macbook, the system default is the discrete GPU.