Fix race condition in ART configuration script

[ketanri]

Description

This addresses a race condition I found when running multiple scripts concurrently which all import ART. Namely, in the config.py file, since there were two separate checks for if os.path.exists(_config_path) and if not os.path.exists(_config_path), the race condition occurs if another process creates the file at _config_path in between those two checks, which can leave the _config variable uninitialized. Hence, I changed those two separate checks into an if/else.

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.

• On my own repo, the issue stopped occurring when pointing the adversarial-robustness-toolbox package to my fork with this fix.

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 #2132 (f6a9d2d) into dev_1.15.0 (b13f802) will decrease coverage by 1.38%.
The diff coverage is 50.00%.

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@@              Coverage Diff               @@
##           dev_1.15.0    #2132      +/-   ##
==============================================
- Coverage       85.63%   84.26%   -1.38%     
==============================================
  Files             297      297              
  Lines           26521    26521              
  Branches         4864     4863       -1     
==============================================
- Hits            22712    22347     -365     
- Misses           2571     2927     +356     
- Partials         1238     1247       +9     

Impacted Files	Coverage Δ
art/config.py	92.85% <50.00%> (-7.15%)	⬇️

... and 15 files with indirect coverage changes