Rename graphs.py to visualizations.py

[eccabay]

Final step of the code side of the graphs.py overhaul (docs updates coming next). graphs.py has been renamed, and now only contains functions that are visualization-specific model understanding functions.

Happy to move more functions around if parts of this organization don't make sense.

[codecov]

Codecov Report

Merging #3439 (47f41f4) into main (40b7e34) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3439     +/-   ##
=======================================
+ Coverage   99.7%   99.7%   +0.1%     
=======================================
  Files        334     334             
  Lines      32951   32959      +8     
=======================================
+ Hits       32821   32829      +8     
  Misses       130     130             

Impacted Files	Coverage Δ
evalml/model_understanding/visualizations.py	100.0% <ø> (ø)
evalml/model_understanding/__init__.py	100.0% <100.0%> (ø)
...odel_understanding/partial_dependence_functions.py	100.0% <100.0%> (ø)
...alml/model_understanding/permutation_importance.py	100.0% <100.0%> (ø)
...understanding_tests/test_permutation_importance.py	100.0% <100.0%> (ø)
...s/model_understanding_tests/test_visualizations.py	100.0% <100.0%> (ø)
evalml/tests/utils_tests/test_gen_utils.py	100.0% <100.0%> (ø)

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[chukarsten]

LGTM! Thanks a lot for completing this!