Allow passing graph level attributes to graphviz

[zaxtax]

This add the ability to pass in graph level attributes to the model_to_graphviz function. This makes it easier to tweak the layout of the graph.

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

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📚 Documentation preview 📚: https://pymc--7741.org.readthedocs.build/en/7741/

[jessegrabowski]

Not sure I like the custom type, it makes it seem more complex than it really is. It's not much shorter than dict[str, Any]

[zaxtax]

I have no strong feelings about this other than it was to be more consistent with the rest of the file

[jessegrabowski]

up to you; not a blocker

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.82%. Comparing base (fb4b201) to head (dcf2b76).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7741   +/-   ##
=======================================
  Coverage   92.82%   92.82%           
=======================================
  Files         107      107           
  Lines       18324    18324           
=======================================
  Hits        17010    17010           
  Misses       1314     1314           

Files with missing lines	Coverage Δ
pymc/model_graph.py	87.25% <100.00%> (ø)

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