Remove conversion to RGB (#6479)

* Remove conversion to RGB

* Add a Conversion Function

* Add type hint for convert_method

* Update src/diffusers/utils/loading_utils.py

Update docstring
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

* Update docstring

* Optimize imports

* Optimize imports (2)

* Reformat code

---------
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

src/diffusers/utils/loading_utils.py

 import os
-from typing import Union
+from typing import Callable, Union
 
 import PIL.Image
 import PIL.ImageOps
 import requests
 
 
-def load_image(image: Union[str, PIL.Image.Image]) -> PIL.Image.Image:
+def load_image(
+    image: Union[str, PIL.Image.Image], convert_method: Callable[[PIL.Image.Image], PIL.Image.Image] = None
+) -> PIL.Image.Image:
     """
     Loads `image` to a PIL Image.
 
     Args:
         image (`str` or `PIL.Image.Image`):
             The image to convert to the PIL Image format.
+        convert_method (Callable[[PIL.Image.Image], PIL.Image.Image], optional):
+            A conversion method to apply to the image after loading it.
+            When set to `None` the image will be converted "RGB".
+
     Returns:
         `PIL.Image.Image`:
             A PIL Image.
@@ -24,14 +30,18 @@ def load_image(image: Union[str, PIL.Image.Image]) -> PIL.Image.Image:
             image = PIL.Image.open(image)
         else:
             raise ValueError(
-                f"Incorrect path or url, URLs must start with `http://` or `https://`, and {image} is not a valid path"
+                f"Incorrect path or URL. URLs must start with `http://` or `https://`, and {image} is not a valid path."
             )
-    elif isinstance(image, PIL.Image.Image):
-        image = image
     else:
         raise ValueError(
-            "Incorrect format used for image. Should be an url linking to an image, a local path, or a PIL image."
+            "Incorrect format used for the image. Should be a URL linking to an image, a local path, or a PIL image."
         )
+
     image = PIL.ImageOps.exif_transpose(image)
-    image = image.convert("RGB")
+
+    if convert_method is not None:
+        image = convert_method(image)
+    else:
+        image = image.convert("RGB")
+
     return image