Bug fix sleeper agent

[monshri]

Description

When the loss is nan or inf, then best_x_poisoned is never assigned, leading to the error mentioned in

• https://github.com/Trusted-AI/adversarial-robustness-toolbox/issues/1865

Fixes # (1865)

Type of change

Please check all relevant options.

• Improvement (non-breaking)
• Bug fix (non-breaking)
• New feature (non-breaking)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Testing

Please describe the tests that you ran to verify your changes. Consider listing any relevant details of your test configuration.

• Test A
• Test B

Test Configuration:

• OS
• Python version
• ART version or commit number
• TensorFlow / Keras / PyTorch / MXNet version

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov-commenter]

Codecov Report

Merging #1892 (efffe13) into dev_1.12.2 (dc29bbc) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@              Coverage Diff               @@
##           dev_1.12.2    #1892      +/-   ##
==============================================
- Coverage       85.85%   85.83%   -0.02%     
==============================================
  Files             248      248              
  Lines           23312    23317       +5     
  Branches         4212     4213       +1     
==============================================
  Hits            20015    20015              
- Misses           2232     2236       +4     
- Partials         1065     1066       +1     

Impacted Files	Coverage Δ
art/attacks/poisoning/sleeper_agent_attack.py	40.30% <0.00%> (-1.06%)	⬇️

[lgtm-com]

This pull request introduces 2 alerts when merging efffe13 into dc29bbc - view on LGTM.com

new alerts:

• 2 for Wrong name for an argument in a call

[lgtm-com]

This pull request introduces 2 alerts when merging f6e7c2c into dc29bbc - view on LGTM.com

new alerts:

• 2 for Wrong name for an argument in a call

[beat-buesser]

Hi @monshri Thank you very much! It looks good to me.