Add type hints to utils.py.

[bigbird555]

Motivation

Please refer to #711 .

Description of the changes

I added some type hints to optuna/visualization/utils.py

[codecov-commenter]

Codecov Report

Merging #1270 into master will decrease coverage by 0.28%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1270      +/-   ##
==========================================
- Coverage   87.01%   86.73%   -0.29%     
==========================================
  Files          94       95       +1     
  Lines        7233     7432     +199     
==========================================
+ Hits         6294     6446     +152     
- Misses        939      986      +47     

Impacted Files	Coverage Δ
optuna/visualization/utils.py	87.50% <100.00%> (+9.72%)	⬆️
optuna/cli.py	27.27% <0.00%> (-4.78%)	⬇️
optuna/samplers/cmaes.py	77.16% <0.00%> (-2.57%)	⬇️
optuna/samplers/tpe/sampler.py	86.22% <0.00%> (-2.00%)	⬇️
optuna/importance/_fanova.py	86.30% <0.00%> (-0.85%)	⬇️
optuna/storages/rdb/storage.py	95.16% <0.00%> (-0.67%)	⬇️
optuna/integration/chainermn.py	75.41% <0.00%> (-0.29%)	⬇️
optuna/storages/cached_storage.py	95.63% <0.00%> (-0.20%)	⬇️
optuna/exceptions.py	100.00% <0.00%> (ø)
optuna/pruners/nop.py	75.00% <0.00%> (ø)
... and 18 more

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[keisuke-umezawa]

LGTM!