Notebook: auto batch size, gradient checkpointing, no more OOM

Fix notebook: use pure-parquet datasets only, cartoon default

Fix notebook: use open SDXL VAE (no auth), 4ch latents, correct scaling

Update notebook: latent pre-caching + small dataset presets for Colab

Fix: SDXL VAE (no login) + streaming dataset in notebook

Fix: streaming dataset in notebook (no full download on Colab)

Add Colab training notebook