Remove incorrectly duplicated queries in SignOPT attack

[Lodour]

Description

The current implementation of SignOPT sends duplicated queries when checking targeted/untargeted modes. This is because the model query happens before the mode checking. For example:

adversarial-robustness-toolbox/art/attacks/evasion/sign_opt.py

Lines 309 to 311 in 22dbf83

	if (not self._is_label(x_0 + lbd * theta, target) and self.targeted) or (
	self._is_label(x_0 + lbd * theta, y_0) and not self.targeted
	):

In untargeted mode, self.targeted = False would cost an unnecessary self._is_label call.

This PR moves the mode checks ahead of the self._is_label as a valid short-circuiting.

It is hard to determine the exact number of calls wasted due to this problem, but this PR should reduce half of the running time for this specific part.

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

[beat-buesser]

Hi @Lodour Thank you very much for your pull request! We'll review it as soon as possible.

[codecov-commenter]

Codecov Report

Merging #2129 (4a9ef03) into dev_1.15.0 (b13f802) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

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@@              Coverage Diff               @@
##           dev_1.15.0    #2129      +/-   ##
==============================================
- Coverage       85.63%   85.61%   -0.03%     
==============================================
  Files             297      297              
  Lines           26521    26521              
  Branches         4864     4864              
==============================================
- Hits            22712    22706       -6     
- Misses           2571     2576       +5     
- Partials         1238     1239       +1     

Impacted Files	Coverage Δ
art/attacks/evasion/sign_opt.py	86.44% <100.00%> (-0.37%)	⬇️

... and 3 files with indirect coverage changes

[beat-buesser]

Hi @Lodour Thank you very much for your pull request! The improvements look good to me.