Add aspect ratio bucketing to training scripts

**Is your feature request related to a problem? Please describe.**
When fine tuning SDXL, images are required to be a fixed size (1024x1024) which involves a lot of cropping that both takes time/resources, and often causes important parts of the image to get cropped out, which lowers model quality.

**Describe the solution you'd like.**
The ideal solution would be a simple option for user to enable aspect ratio bucketing (e.g. a command argument `--enable-bucketing`) that will let them train with multiple image sizes