Fix Image Poisoning Perturbations Trigger Placement Bug

[f4str]

Description

Fix the bugs involving the trigger placement for image poisoning perturbations.

1. The dimensions for the trigger height and width are swapped to accommodate for PIL. Now non-square triggers placed on the input image will be the correct dimension.
2. The off-by-one error for the random placement was addressed. Now triggers of the same dimension as the image will work when the random parameter is set to true.

Additionally, apply minor optimizations for better performance and readability.

Fixes #2141

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.

• Image perturbation tests

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 #2143 (90350ef) into dev_1.15.0 (b13f802) will decrease coverage by 0.01%.
The diff coverage is 66.66%.

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@@              Coverage Diff               @@
##           dev_1.15.0    #2143      +/-   ##
==============================================
- Coverage       85.63%   85.63%   -0.01%     
==============================================
  Files             297      297              
  Lines           26521    26521              
  Branches         4864     4864              
==============================================
- Hits            22712    22710       -2     
- Misses           2571     2572       +1     
- Partials         1238     1239       +1     

Impacted Files	Coverage Δ
...cks/poisoning/perturbations/image_perturbations.py	93.58% <66.66%> (ø)

... and 1 file with indirect coverage changes

[beat-buesser]

Hi @f4str Thank you very much! The solution looks good to me.