Refactor Mac specific code to a separate file

Move most Mac related code to a separate file, don't even load it unless web UI is run under macOS.

modules/shared.py

@@ -145,6 +145,9 @@ devices.device, devices.device_interrogate, devices.device_gfpgan, devices.devic
     (devices.cpu if any(y in cmd_opts.use_cpu for y in [x, 'all']) else devices.get_optimal_device() for x in ['sd', 'interrogate', 'gfpgan', 'esrgan', 'codeformer'])
 
 device = devices.device
+if sys.platform == "darwin":
+    from modules import mac_specific
+    mac_specific.device = device
 weight_load_location = None if cmd_opts.lowram else "cpu"
 
 batch_cond_uncond = cmd_opts.always_batch_cond_uncond or not (cmd_opts.lowvram or cmd_opts.medvram)