fix sde add noise typo (huggingface#3839)

* fix sde typo

* fix code style

Author: UranusITS

src/diffusers/schedulers/scheduling_sde_ve.py

@@ -276,7 +276,11 @@ def add_noise(
         # Make sure sigmas and timesteps have the same device and dtype as original_samples
         timesteps = timesteps.to(original_samples.device)
         sigmas = self.discrete_sigmas.to(original_samples.device)[timesteps]
-        noise = torch.randn_like(original_samples) * sigmas[:, None, None, None]
+        noise = (
+            noise * sigmas[:, None, None, None]
+            if noise is not None
+            else torch.randn_like(original_samples) * sigmas[:, None, None, None]
+        )
         noisy_samples = noise + original_samples
         return noisy_samples