[Feature Request]Use Gzip for compress latent file size

from reddit by joycaption auther:
https://old.reddit.com/r/StableDiffusion/comments/1gdkpqp/the_gory_details_of_finetuning_sdxl_for_40m/

# Each image in the dataset is about 1MB, which means the dataset as a whole is nearly 7TB, making it infeasible for me to do training in the cloud where I can utilize larger machines. But once gzipped, the latents are only about 100KB each, 10% the size, dropping it to 725GB for the whole dataset. Much more manageable. (Note: I tried zstandard to see if it could compress further, but it resulted in worse compression ratios even at higher settings. Need to investigate.)

No files, tests, or entry points are named. Start by locating the latent dataset serialization and loading paths, then determine how gzip could fit alongside the existing format; done means training can use compressed latents and the storage reduction is verified.

Written by the indexing model from the issue text.