[OmegaConf] replace it with `yaml` (#6488)
* remove omegaconf from convert_from_ckpt.
* remove from single_file.
* change to string based ubscription.
* style
* okay
* fix: vae_param
* no . indexing.
* style
* style
* turn getattrs into explicit if/else
* style
* propagate changes to ldm_uncond.
* propagate to gligen
* propagate to if.
* fix: quotes.
* propagate to audioldm.
* propagate to audioldm2
* propagate to musicldm.
* propagate to vq_diffusion
* propagate to zero123.
* remove omegaconf from diffusers codebase.
