Fix CogVideoX scheduler prev_timestep for non-leading spacing

src/diffusers/schedulers/scheduling_ddim_cogvideox.py

@@ -383,7 +383,7 @@ def step(
         # - pred_prev_sample -> "x_t-1"
 
         # 1. get previous step value (=t-1)
-        prev_timestep = timestep - self.config.num_train_timesteps // self.num_inference_steps
+        prev_timestep = self.previous_timestep(timestep)
 
         # 2. compute alphas, betas
         alpha_prod_t = self.alphas_cumprod[timestep]
@@ -500,5 +500,24 @@ def get_velocity(self, sample: torch.Tensor, noise: torch.Tensor, timesteps: tor
         velocity = sqrt_alpha_prod * noise - sqrt_one_minus_alpha_prod * sample
         return velocity
 
+    def previous_timestep(self, timestep: int) -> torch.Tensor:
+        """
+        Find the previous timestep in the scheduler's timestep schedule.
+
+        Args:
+            timestep (`int`):
+                The current timestep.
+
+        Returns:
+            `torch.Tensor`:
+                The previous timestep. Returns -1 if the current timestep is the last one.
+        """
+        index = (self.timesteps == timestep).nonzero(as_tuple=True)[0][0]
+        if index == self.timesteps.shape[0] - 1:
+            prev_t = torch.tensor(-1)
+        else:
+            prev_t = self.timesteps[index + 1]
+        return prev_t
+
     def __len__(self) -> int:
         return self.config.num_train_timesteps

src/diffusers/schedulers/scheduling_dpm_cogvideox.py

@@ -467,7 +467,7 @@ def step(
         # - pred_prev_sample -> "x_t-1"
 
         # 1. get previous step value (=t-1)
-        prev_timestep = timestep - self.config.num_train_timesteps // self.num_inference_steps
+        prev_timestep = self.previous_timestep(timestep)
 
         # 2. compute alphas, betas
         alpha_prod_t = self.alphas_cumprod[timestep]
@@ -599,5 +599,24 @@ def get_velocity(self, sample: torch.Tensor, noise: torch.Tensor, timesteps: tor
         velocity = sqrt_alpha_prod * noise - sqrt_one_minus_alpha_prod * sample
         return velocity
 
+    def previous_timestep(self, timestep: int) -> torch.Tensor:
+        """
+        Find the previous timestep in the scheduler's timestep schedule.
+
+        Args:
+            timestep (`int`):
+                The current timestep.
+
+        Returns:
+            `torch.Tensor`:
+                The previous timestep. Returns -1 if the current timestep is the last one.
+        """
+        index = (self.timesteps == timestep).nonzero(as_tuple=True)[0][0]
+        if index == self.timesteps.shape[0] - 1:
+            prev_t = torch.tensor(-1)
+        else:
+            prev_t = self.timesteps[index + 1]
+        return prev_t
+
     def __len__(self) -> int:
         return self.config.num_train_timesteps