Controlnet stack support for models without baked in vae

[DavidJaeck]

If you try to use a controlnet without a vae baked in you get the error:

This Controlnet needs a VAE but none was provided, please use a ControlNetApply node with a VAE input and connect it.

Actually I have implemented support for optionally loading an external vae on my machine, so I could share the implementation. I rarely contribute to open source, I think if I shall open a merge request I need to be granted some permissions. Otherwise I could share the diff in this issue.