Add support for arbitrary-shaped input in APGD and ACG attacks

[beat-buesser]

Description

This pull request removes assumptions of input shape in APGD and ACG evasion attacks.

Fixes #2165

[codecov-commenter]

Codecov Report

Merging #2214 (7aa1f1b) into dev_1.15.1 (c6da8c4) will increase coverage by 8.97%.
The diff coverage is 100.00%.

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@@              Coverage Diff               @@
##           dev_1.15.1    #2214      +/-   ##
==============================================
+ Coverage       76.63%   85.61%   +8.97%     
==============================================
  Files             308      308              
  Lines           27450    27450              
  Branches         5045     5045              
==============================================
+ Hits            21036    23500    +2464     
+ Misses           5227     2669    -2558     
- Partials         1187     1281      +94     

Files Changed	Coverage Δ
art/attacks/evasion/auto_conjugate_gradient.py	86.52% <100.00%> (ø)
...attacks/evasion/auto_projected_gradient_descent.py	86.51% <100.00%> (ø)

... and 40 files with indirect coverage changes

[Zaid-Hameed]

Thanks Beat for the changes!