Add tutorial using classifier-free guidance during the training of a diffusion model or latent diffusion model

[Warvito]

Create a tutorial of a conditioned diffusion model (or latent diffusion model) that includes classifier free guidance. The tutorial should use the MONAI transformations to drop the conditioning during training 10~20% of the time. During inference, it should show example of conditioned sample and possibly a sampling process including negative prompt (https://github.com/AUTOMATIC1111/stable-diffusion-webui/wiki/Negative-prompt).