generate_graphs.py incorrect dependency.

[SamuelLarkin]

I followed the sockeye/sockeye_contrib/vistools/README.md to try generating beam search graphs but stumbled on the following error after installing the latest networkx as instructed by requirements.txt.

python /project/WMT20/opt/sockeye/sockeye_contrib/vistools/generate_graphs.py -d beams.json -o generated_graphs
Traceback (most recent call last):
  File "/project/WMT20/opt/sockeye/sockeye_contrib/vistools/generate_graphs.py", line 158, in <module>
    main()
  File "/project/WMT20/opt/sockeye/sockeye_contrib/vistools/generate_graphs.py", line 154, in main
    generate(args.data, args.output_dir, include_pad=args.include_pad)
  File "/project/WMT20/opt/sockeye/sockeye_contrib/vistools/generate_graphs.py", line 129, in generate
    include_pad=include_pad)
  File "/project/WMT20/opt/sockeye/sockeye_contrib/vistools/generate_graphs.py", line 106, in create_graph
    scores[level], normalized_scores[level], include_pad)
  File "/project/WMT20/opt/sockeye/sockeye_contrib/vistools/generate_graphs.py", line 92, in _add_graph_level
    graph.node[new_node]["name"] = names[i]
AttributeError: 'DiGraph' object has no attribute 'node'

Based on the date on that README.md, I simply reverted networkx to a version close to that date and chose networkx==2.0. I haven't tried any other versions as this one worked.

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[fhieber]

Thanks!