add lines between code block

Author: lezwon

pytorch_lightning/core/lightning.py

@@ -1728,10 +1728,12 @@ def to_onnx(self, file_path: str, input_sample: Optional[Tensor] = None, **kwarg
 
         Example:
             .. code-block:: python
+
                 file_path = './model.onnx'
                 model = MyLightningModule(...)
                 input_sample = torch.randn((1, 28 * 28))
                 model.to_onnx(file_path, input_sample, export_params=True)
+
         """
 
         if isinstance(input_sample, Tensor):