Example `plot_spn.py` raises an error

[remilvus]

I was trying to run plot_spn.py, but the code raises an error. Here's the output:

Plotting the dummy SPN to spn-dummy.svg ...
Traceback (most recent call last):
  File ".../deeprob-kit/examples/spn_plot.py", line 25, in <module>
    spn.plot_spn(root, spn_filename)
  File ".../miniconda3/envs/deeprob/lib/python3.9/site-packages/deeprob/spn/structure/io.py", line 317, in plot_spn
    pos = nx_pydot.graphviz_layout(graph, prog='dot')
  File ".../miniconda3/envs/deeprob/lib/python3.9/site-packages/networkx/drawing/nx_pydot.py", line 357, in graphviz_layout
    return pydot_layout(G=G, prog=prog, root=root)
  File ".../miniconda3/envs/deeprob/lib/python3.9/site-packages/networkx/drawing/nx_pydot.py", line 406, in pydot_layout
    P = to_pydot(G)
  File ".../miniconda3/envs/deeprob/lib/python3.9/site-packages/networkx/drawing/nx_pydot.py", line 263, in to_pydot
    raise ValueError(
ValueError: Node names and attributes should not contain ":" unless they are quoted with "".                For example the string 'attribute:data1' should be written as '"attribute:data1"'.                Please refer https://github.com/pydot/pydot/issues/258

[janakact]

Facing the same error.