[doc] update inpaint doc to use `apply_overlay` (#6364)

add doc
Co-authored-by: yiyixuxu <yixu310@gmail,com>

docs/source/en/using-diffusers/inpaint.md

@@ -318,7 +318,7 @@ make_image_grid([init_image, image], rows=1, cols=2)
 
 The trade-off of using a non-inpaint specific checkpoint is the overall image quality may be lower, but it generally tends to preserve the mask area (that is why you can see the mask outline). The inpaint specific checkpoints are intentionally trained to generate higher quality inpainted images, and that includes creating a more natural transition between the masked and unmasked areas. As a result, these checkpoints are more likely to change your unmasked area.
 
-If preserving the unmasked area is important for your task, you can use the code below to force the unmasked area of an image to remain the same at the expense of some more unnatural transitions between the masked and unmasked areas.
+If preserving the unmasked area is important for your task, you can use the `apply_overlay` method of [`VaeImageProcessor`] to force the unmasked area of an image to remain the same at the expense of some more unnatural transitions between the masked and unmasked areas.
 
 ```py
 import PIL
@@ -345,18 +345,7 @@ prompt = "Face of a yellow cat, high resolution, sitting on a park bench"
 repainted_image = pipeline(prompt=prompt, image=init_image, mask_image=mask_image).images[0]
 repainted_image.save("repainted_image.png")
 
-# Convert mask to grayscale NumPy array
-mask_image_arr = np.array(mask_image.convert("L"))
-# Add a channel dimension to the end of the grayscale mask
-mask_image_arr = mask_image_arr[:, :, None]
-# Binarize the mask: 1s correspond to the pixels which are repainted
-mask_image_arr = mask_image_arr.astype(np.float32) / 255.0
-mask_image_arr[mask_image_arr < 0.5] = 0
-mask_image_arr[mask_image_arr >= 0.5] = 1
-
-# Take the masked pixels from the repainted image and the unmasked pixels from the initial image
-unmasked_unchanged_image_arr = (1 - mask_image_arr) * init_image + mask_image_arr * repainted_image
-unmasked_unchanged_image = PIL.Image.fromarray(unmasked_unchanged_image_arr.round().astype("uint8"))
+unmasked_unchanged_image = pipeline.image_processor.apply_overlay(mask_image, init_image, repainted_image)
 unmasked_unchanged_image.save("force_unmasked_unchanged.png")
 make_image_grid([init_image, mask_image, repainted_image, unmasked_unchanged_image], rows=2, cols=2)
 ```