[Feature] Preserve Resolution option for datasets

[enlyth]

Sometimes you might be in a situation where you want to train on your original dataset resolutions without resizing, because you have either pre-resized your dataset, or it has specific requirements that resizing would be detrimental to (such as training pixel-art datasets).

This feature adds a preserve_resolutions boolean option to datasets, defaulting to False, which if True, completely disables resizing.

It basically results in one bucket per each unique image resolution in your dataset, regardless of what it is, without resizing.