Return "None" when dspy.Image is missing for the input, similarly to str input fields

[rogeriochaves]

when using the new dspy.Image field, it's not possible to make it optional (for example if you have 2-3 images as input, but not all of them are required). For str input fields, that's not a problem, as None inputs are just rendered as a literal "None" on the prompt for the LMs

simple example, this breaks for the image field, but not for the description field:

class ImageReader(dspy.Signature):
    """What is in the image?"""

    image: dspy.Image = dspy.InputField()
    description: str = dspy.InputField()
    response: str = dspy.OutputField()


dspy.Predict(ImageReader)(image=None, description=None)

I propose then simply returning "None", explicitly as a text type, so the LM knows clearly that the input is lacking an image

[okhat]

Hey @rogeriochaves !! Can one do typing.Optional[dspy.Image]? I'm guessing that's not supported right now, right?