Update utils.py

[nmegiddo]

Use a different algorithm for norm=1, based on exponential distribution.

Description

A sample point is generated by a L1-normalized vector (in dimenion nb_dims + 1) of IID unit-exponential variables, then dropping the additional dimension and flipping signs randomly.

Fixes # (issue)

Type of change

Please check all relevant options.

• [v] Improvement (non-breaking)
• [v] 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 #1805 (aa8f4d3) into dev_1.12.0 (5c97a8a) will increase coverage by 0.05%. The diff coverage is 100.00%.

@@              Coverage Diff               @@
##           dev_1.12.0    #1805      +/-   ##
==============================================
+ Coverage       85.39%   85.44%   +0.05%     
==============================================
  Files             274      274              
  Lines           23857    23858       +1     
  Branches         4262     4261       -1     
==============================================
+ Hits            20373    20386      +13     
+ Misses           2422     2411      -11     
+ Partials         1062     1061       -1     

Impacted Files	Coverage Δ
art/utils.py	75.33% <100.00%> (+0.04%)	:arrow_up:
art/estimators/certification/abstain.py	90.90% <0.00%> (-9.10%)	:arrow_down:
...poison_mitigation/neural_cleanse/neural_cleanse.py	84.21% <0.00%> (-6.15%)	:arrow_down:
art/estimators/poison_mitigation/strip/strip.py	94.44% <0.00%> (-5.56%)	:arrow_down:
art/defences/detector/poison/activation_defence.py	83.23% <0.00%> (-0.29%)	:arrow_down:
art/attacks/evasion/sign_opt.py	86.86% <0.00%> (+0.36%)	:arrow_up:
art/attacks/evasion/boundary.py	93.97% <0.00%> (+1.20%)	:arrow_up:
...timators/poison_mitigation/neural_cleanse/keras.py	92.36% <0.00%> (+13.88%)	:arrow_up: