How to provide a target and input separately for Trainer?

I want to fine-tune a GPT2 model for a specific style of text generation. For this I want to provide it with some text, but I don’t want the model to learn to generate the full text, I only want it to learn to generate the last part if given the first part. How do I do this with a trainer?

In principle it seems to me that this should be trivial to do and has many use-cases, but I can’t find a way to do it.

Example:

text = “”"
I want to destroy the world! # I don’t want my model to learn to generate text like this.
No, that’s bad, don’t do that! # I want my model to learn to generate text like this if given the first part to reply to.
“”"