Allow serialize to gather implicit nodes

[liamgriffiths]

https://github.com/orgs/SubstrateLabs/projects/1/views/3?pane=issue&itemId=66030127

Allows us to only need to pass in the terminal nodes into substrate.run (and other methods that use serialize)

scene = GenerateText(prompt="description of a mythical forest creature: ")

styles = ["woodblock printed", "art nouveau poster"]
images = [GenerateImage(store="hosted", prompt=sb.concat(style, ": ", scene.future.text)) for style in styles]

result = substrate.run(*images)