[Fix] Add Support for Images not Div by 16 (Diff. JPEG)

[ChristophReich1996]

This pull request adds support for images not divisible by 16 to differentiable JPEG coding. As discussed in #2845 the current differentiable JPEG implementation just supports images that are divisible by 16. This is due to the fact that JPEG performs a (8 x 8) patch-wise DCT on downsampled (by 2) images. Thus the image must be divisible by 16. This pull request adds support for arbitrarily shaped images by performing padding and cropping. This implementation follows the common padding approach used before coding. In particular, the bottom and right sides of the image are padded by replication (see slide 442 of this reference). To go back to the original shape the JPEG-coded image is simply cropped again.

Type of change

• 📚 Documentation Update
• 🧪 Tests Cases
• 🐞 Bug fix (non-breaking change which fixes an issue)
• 🔬 New feature (non-breaking change which adds functionality)
• 🚨 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, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Did you update CHANGELOG in case of a major change?