import torch missing in stable-diffusion-2 usage snippet

Just imported the torch package in the example usage code snippet so it can be copied as-is and run without errors.

README.md

@@ -45,6 +45,7 @@ pip install diffusers transformers accelerate scipy safetensors
 Running the pipeline (if you don't swap the scheduler it will run with the default DDIM, in this example we are swapping it to EulerDiscreteScheduler):
 
 ```python
+import torch
 from diffusers import StableDiffusionPipeline, EulerDiscreteScheduler
 
 model_id = "stabilityai/stable-diffusion-2"