TensorRT assert im.device.type != 'cpu' on export (#6340)

* TensorRT `assert im.device.type != 'cpu'` on export

* Update export.py

export.py

@@ -184,9 +184,10 @@ def export_engine(model, im, file, train, half, simplify, workspace=4, verbose=F
             check_version(trt.__version__, '8.0.0', hard=True)  # require tensorrt>=8.0.0
             export_onnx(model, im, file, 13, train, False, simplify)  # opset 13
         onnx = file.with_suffix('.onnx')
-        assert onnx.exists(), f'failed to export ONNX file: {onnx}'
 
         LOGGER.info(f'\n{prefix} starting export with TensorRT {trt.__version__}...')
+        assert im.device.type != 'cpu', 'export running on CPU but must be on GPU, i.e. `python export.py --device 0`'
+        assert onnx.exists(), f'failed to export ONNX file: {onnx}'
         f = file.with_suffix('.engine')  # TensorRT engine file
         logger = trt.Logger(trt.Logger.INFO)
         if verbose: