Disallow `--dynamic` when `--half` is passed (#7669)

* disallow dynamic arg when half is given

* Update export.py

Co-authored-by: David Matos <david@track32.nl>
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

export.py

@@ -478,6 +478,7 @@ def run(
     device = select_device(device)
     if half:
         assert device.type != 'cpu' or coreml or xml, '--half only compatible with GPU export, i.e. use --device 0'
+        assert not dynamic, '--half not compatible with --dynamic, i.e. use either --half or --dynamic but not both'
     model = attempt_load(weights, map_location=device, inplace=True, fuse=True)  # load FP32 model
     nc, names = model.nc, model.names  # number of classes, class names