fix: import os before updating sys.path in DIT latent script

[peter941221]

Bug

create_imagenet_latents.py appends to sys.path with os.path.join(...) before import os.

On current main, that crashes at import time with:
NameError: name 'os' is not defined

vae_hf_cs.py already fixed the same import-order bug, but this DIT latent script still has the broken order.

Fix

Move import os above the sys.path.append(...) call in create_imagenet_latents.py.

This keeps the script behavior the same apart from removing the import-time failure.

Verification

On origin/main:
python3 src/cerebras/modelzoo/data_preparation/vision/dit/create_imagenet_latents.py

fails immediately with:
NameError: name 'os' is not defined

On this branch, the same command gets past that line and fails later in the current environment with:
ModuleNotFoundError: No module named 'torch'

I also ran:
python3 -m py_compile src/cerebras/modelzoo/data_preparation/vision/dit/create_imagenet_latents.py