Fix bug in PixelThreshold for single input

[aufieroma12]

Description

Calling .generate() on a batch of length one, a TypeError is raised on line 193.

TypeError: iteration over a 0-d array

This is because np.squeeze(y) returns a 0-dimensional array when y.shape == (1, 1). Changing it to y.flatten() solves this issue by returning a 1-dimensional array in this case, while not changing the behavior in the case when the batch has more than one input.

Fixes # (issue)

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.

• Ran Pixel Attack on input array of shape (1, 3, 224, 224) (this did not work before, and it does not)
• Ran Pixel Attack on input array of shape (3, 3, 224, 224) (this still works as before)

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