Correct type annotation for VaeImageProcessor.numpy_to_pil (#6111)

From `(np.ndarray) -> PIL.Image.Image` to `(np.ndarray) -> List[PIL.Image.Image]`.

Author: edwardwli

src/diffusers/image_processor.py

@@ -88,7 +88,7 @@ def __init__(
             self.config.do_convert_rgb = False
 
     @staticmethod
-    def numpy_to_pil(images: np.ndarray) -> PIL.Image.Image:
+    def numpy_to_pil(images: np.ndarray) -> List[PIL.Image.Image]:
         """
         Convert a numpy image or a batch of images to a PIL image.
         """