Update README.md

Add missing imports, model_id and device to float16 sample code.

README.md

@@ -90,6 +90,11 @@ If you are limited by GPU memory and have less than 10GB of GPU RAM available, p
 
 ```py
 import torch
+from torch import autocast
+from diffusers import StableDiffusionPipeline
+
+model_id = "CompVis/stable-diffusion-v1-4"
+device = "cuda"
 
 pipe = StableDiffusionPipeline.from_pretrained(model_id, torch_dtype=torch.float16, revision="fp16", use_auth_token=True)
 pipe = pipe.to(device)