Update check_and_transform_label_format for index labels

[beat-buesser]

Description

Fix missing transformation of binary index to one-hot encoded labels in art.utils.check_and_transform_label_format.

Fixes #1334

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

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 #1443 (e594795) into dev_1.9.0 (df2e613) will increase coverage by 0.00%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           dev_1.9.0    #1443   +/-   ##
==========================================
  Coverage      90.30%   90.31%           
==========================================
  Files            240      240           
  Lines          19679    19682    +3     
  Branches        3490     3490           
==========================================
+ Hits           17771    17775    +4     
+ Misses          1112     1110    -2     
- Partials         796      797    +1     

Impacted Files	Coverage Δ
art/attacks/evasion/pixel_threshold.py	91.21% <50.00%> (+0.04%)	⬆️
art/utils.py	91.15% <90.00%> (+0.22%)	⬆️
art/estimators/classification/keras.py	88.41% <100.00%> (+0.70%)	⬆️
art/defences/detector/poison/activation_defence.py	84.84% <0.00%> (-1.35%)	⬇️
art/estimators/classification/pytorch.py	89.19% <0.00%> (+0.55%)	⬆️