fix bug in dpm_solver_first_order_update

src/refiners/foundationals/latent_diffusion/schedulers/dpm_solver.py

@@ -51,7 +51,7 @@ def _generate_timesteps(self) -> Tensor:
 
     def dpm_solver_first_order_update(self, x: Tensor, noise: Tensor, step: int) -> Tensor:
         current_timestep = self.timesteps[step]
-        previous_timestep = self.timesteps[step + 1 if step < len(self.timesteps) - 1 else 0]
+        previous_timestep = self.timesteps[step + 1] if step < self.num_inference_steps - 1 else tensor([0])
 
         previous_ratio = self.signal_to_noise_ratios[previous_timestep]
         current_ratio = self.signal_to_noise_ratios[current_timestep]