Update README.md code to import `torch`

The sample code snippet doesn't run by itself because `torch` would be undefined in
```python
StableDiffusionPipeline.from_pretrained(model_id, scheduler=scheduler, torch_dtype=torch.float16)
```

README.md

@@ -46,6 +46,7 @@ Running the pipeline (if you don't swap the scheduler it will run with the defau
 
 ```python
 from diffusers import StableDiffusionPipeline, EulerDiscreteScheduler
+import torch
 
 model_id = "stabilityai/stable-diffusion-2"