Fix test models to output logits and work with default loss functions

[jklaise]

Fixes the issue described here: #968 (comment)

I opted to change both models to output logits (for consistency), this requires keeping from_logits=True for the tensorflow loss function (default is False).

[codecov]

Codecov Report

Merging #975 (6aca56d) into master (7f1c2f3) will not change coverage.
Report is 1 commits behind head on master.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #975   +/-   ##
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  Coverage   85.23%   85.23%           
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  Files          74       74           
  Lines        8834     8834           
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  Hits         7530     7530           
  Misses       1304     1304           

Files	Coverage Δ
alibi/explainers/tests/test_simiarlity/conftest.py	100.00% <ø> (ø)