Add type hints to test_keras.py.

[bigbird555]

Motivation

Please refer to the Issue #711 .

Description of the changes

I added type hints to integration_tests/test_keras.py.

[codecov-commenter]

Codecov Report

Merging #1287 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1287   +/-   ##
=======================================
  Coverage   86.70%   86.71%           
=======================================
  Files          95       95           
  Lines        7434     7437    +3     
=======================================
+ Hits         6446     6449    +3     
  Misses        988      988           

Impacted Files	Coverage Δ
optuna/integration/allennlp.py	86.66% <0.00%> (+0.70%)	⬆️

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