stabilize dpmpp for sdxl by using euler at the final step

src/diffusers/schedulers/scheduling_dpmsolver_multistep.py

@@ -117,6 +117,10 @@ class DPMSolverMultistepScheduler(SchedulerMixin, ConfigMixin):
         lower_order_final (`bool`, defaults to `True`):
             Whether to use lower-order solvers in the final steps. Only valid for < 15 inference steps. This can
             stabilize the sampling of DPMSolver for steps < 15, especially for steps <= 10.
+        euler_at_final (`bool`, defaults to `False`):
+            Whether to use Euler's method in the final step. It is a trade-off between numerical stability and detail
+            richness. This can stabilize the sampling of the SDE variant of DPMSolver for small number of inference
+            steps, but sometimes may result in blurring.
         use_karras_sigmas (`bool`, *optional*, defaults to `False`):
             Whether to use Karras sigmas for step sizes in the noise schedule during the sampling process. If `True`,
             the sigmas are determined according to a sequence of noise levels {σi}.
@@ -154,6 +158,7 @@ def __init__(
         algorithm_type: str = "dpmsolver++",
         solver_type: str = "midpoint",
         lower_order_final: bool = True,
+        euler_at_final: bool = False,
         use_karras_sigmas: Optional[bool] = False,
         lambda_min_clipped: float = -float("inf"),
         variance_type: Optional[str] = None,
@@ -787,8 +792,9 @@ def step(
         if self.step_index is None:
             self._init_step_index(timestep)
 
-        lower_order_final = (
-            (self.step_index == len(self.timesteps) - 1) and self.config.lower_order_final and len(self.timesteps) < 15
+        # Improve numerical stability for small number of steps
+        lower_order_final = (self.step_index == len(self.timesteps) - 1) and (
+            self.config.euler_at_final or (self.config.lower_order_final and len(self.timesteps) < 15)
         )
         lower_order_second = (
             (self.step_index == len(self.timesteps) - 2) and self.config.lower_order_final and len(self.timesteps) < 15