Add latent consistency

[patrickvonplaten]

What does this PR do?

This PR adds: https://latent-consistency-models.github.io/ from https://github.com/luosiallen .

@luosiallen feel free to suggest any changes to the wording. Once this PR is merged the following code will work out of the box for existing PyPi versions:

from diffusers import DiffusionPipeline
import torch

pipe = DiffusionPipeline.from_pretrained("SimianLuo/LCM_Dreamshaper_v7", custom_pipeline="latent_consistency_txt2img")
pipe.to(torch_device="cuda", torch_dtype=torch.float16)

prompt = "Self-portrait oil painting, a beautiful cyborg with golden hair, 8k"
num_inference_steps = 4  # can be changed to 1, 2 or 4

images = pipe(prompt=prompt, num_inference_steps=num_inference_steps, guidance_scale=8.0, lcm_origin_steps=50, output_type="pil").images