Allow `safety_checker` to be `None` when using CPU offload (#1078)

Allow None safety_checker when using CPU offload.

src/diffusers/pipelines/stable_diffusion/pipeline_stable_diffusion.py

@@ -133,6 +133,7 @@ class StableDiffusionPipeline(DiffusionPipeline):
         device = torch.device("cuda")
 
         for cpu_offloaded_model in [self.unet, self.text_encoder, self.vae, self.safety_checker]:
+            if cpu_offloaded_model is not None:
                 cpu_offload(cpu_offloaded_model, device)
 
     @torch.no_grad()