fix: Display demo utils gallery with arbitrary number of images

[thomas-coldwell]

What does this PR do?

Hi folks - I noticed that in some of the examples if you set the batch size != 9 then calling demo_utils.visualize_data would raise an error as the gallery_show method expected exactly 9 images as input. This PR now allows it to display an arbitrary number of images with a max of 3 columns (can be changed or turned into a parameter maybe?).

Motivation for this is to make it easier when debugging with any of the examples and you just want to consider the results with a batch size of only 1 or 2 images

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