Load latent file without original image

[sheldonxxxx]

Hi,

I'm using a cloud environment to do training. To minimize the upload file size, I only upload the latent files and the meta_lat.json to the environment, but the current dataset handler does not allow this.

I make the following change to train_util.py:47, and training works fine. IMAGE_EXTENSIONS = [".png", ".jpg", ".jpeg", ".webp", ".bmp", ".npz"]

From my understanding, this change should not break anything. Please consider this use case in your next update, thx.