[fix bug] PixArt inference_steps=1 (huggingface#11079)

* fix bug when pixart-dmd inference with `num_inference_steps=1`

* use return_dict=False and return [1] element for 1-step pixart model, which works for both lcm and dmd

Author: lawrence-cj

src/diffusers/pipelines/pixart_alpha/pipeline_pixart_alpha.py

@@ -941,8 +941,7 @@ def __call__(
 
                 # compute previous image: x_t -> x_t-1
                 if num_inference_steps == 1:
-                    # For DMD one step sampling: https://arxiv.org/abs/2311.18828
-                    latents = self.scheduler.step(noise_pred, t, latents, **extra_step_kwargs).pred_original_sample
+                    latents = self.scheduler.step(noise_pred, t, latents, **extra_step_kwargs, return_dict=False)[1]
                 else:
                     latents = self.scheduler.step(noise_pred, t, latents, **extra_step_kwargs, return_dict=False)[0]