LMInput restricts model input to a single collection of images and video frames

[davidkoski]

See #277 and #276

The UserInput struct can represent a series of messages with media attached to each image:

        return UserInput(
            chat: [
                .system(generate.system),
                .user(prompt, images: media.images, videos: media.videos),
            ],
            processing: media.processing
        )

This could include back and forth between the user and assistant including adding additional media.

The UserInputProcessor converts this to an LMInput:

public struct LMInput {
    public let text: Text
    public let image: ProcessedImage?
    public let video: ProcessedVideo?

but that only allows for one set of image/video. This should probably have:

    public let images: [ProcessedImage]
    public let videos: [ProcessedVideo]

though the model would have to be updated to take advantage of that.

Consider this chat:

> /image /tmp/img.jpeg


> what animal is in the image?
[["role": "system", "content": [["text": "You are a helpful assistant who answers questions in English.", "type": "text"]]], ["role": "user", "content": [["text": "what animal is in the image?", "type": "text"], **["type": "image"]**]]]
The animal in the image is a dog.

> /image /tmp/img2.jpeg


> describe the second image
[["content": [["text": "You are a helpful assistant who answers questions in English.", "type": "text"]], "role": "system"], ["content": [["text": "what animal is in the image?", "type": "text"], **["type": "image"]**], "role": "user"], ["content": [["type": "text", "text": "The animal in the image is a dog."]], "role": "assistant"], ["content": [["type": "text", "text": "describe the second image"], **["type": "image"]**], "role": "user"]]
The image shows a dog wearing a Santa hat.

Ideally this would present the second image for the second image marker. As it is today it will combine both images and inject them for the first marker.

[davidkoski]

@ibrahimcetin FYI

@Blaizzy I think the python mlx-vlm has the same issue (roughly). It doesn't have the same structures but it does treat the media as a single bundle.

Assuming of course it is supposed to work like this!