model.fuse() fix for export.py (#827)

models/export.py

@@ -28,6 +28,9 @@ if __name__ == '__main__':
     attempt_download(opt.weights)
     model = torch.load(opt.weights, map_location=torch.device('cpu'))['model'].float()
     model.eval()
+    model.fuse()
+
+    # Update model
     model.model[-1].export = True  # set Detect() layer export=True
     y = model(img)  # dry run
 
@@ -47,7 +50,6 @@ if __name__ == '__main__':
 
         print('\nStarting ONNX export with onnx %s...' % onnx.__version__)
         f = opt.weights.replace('.pt', '.onnx')  # filename
-        model.fuse()  # only for ONNX
         torch.onnx.export(model, img, f, verbose=False, opset_version=12, input_names=['images'],
                           output_names=['classes', 'boxes'] if y is None else ['output'])