fix heun scheduler (huggingface#1512)

Author: patil-suraj

src/diffusers/schedulers/scheduling_heun.py

@@ -186,10 +186,13 @@ def step(
 
         # 1. compute predicted original sample (x_0) from sigma-scaled predicted noise
         if self.config.prediction_type == "epsilon":
-            pred_original_sample = sample - sigma_hat * model_output
+            sigma_input = sigma_hat if self.state_in_first_order else sigma_next
+            pred_original_sample = sample - sigma_input * model_output
         elif self.config.prediction_type == "v_prediction":
-            # * c_out + input * c_skip
-            pred_original_sample = model_output * (-sigma / (sigma**2 + 1) ** 0.5) + (sample / (sigma**2 + 1))
+            sigma_input = sigma_hat if self.state_in_first_order else sigma_next
+            pred_original_sample = model_output * (-sigma_input / (sigma_input**2 + 1) ** 0.5) + (
+                sample / (sigma_input**2 + 1)
+            )
         else:
             raise ValueError(
                 f"prediction_type given as {self.config.prediction_type} must be one of `epsilon`, or `v_prediction`"
@@ -207,7 +210,7 @@ def step(
             self.sample = sample
         else:
             # 2. 2nd order / Heun's method
-            derivative = (sample - pred_original_sample) / sigma_hat
+            derivative = (sample - pred_original_sample) / sigma_next
             derivative = (self.prev_derivative + derivative) / 2
 
             # 3. Retrieve 1st order derivative