Add import os to vae_hf_cs.py

[vksastry]

The "import os" is missing from https://github.com/Cerebras/modelzoo/blob/main/modelzoo/vision/pytorch/dit/checkpoint_converter/vae_hf_cs.py.

[peter941221]

Opened a draft PR for the remaining sibling DIT import-order bug in create_imagenet_latents.py. Draft PR: #87