Huggingface GPT Seq2Seq input format

I want to finetune a GPT-like model on a Seq2Seq task. Even though I have been using Huggingface for quite some time, I am very confused about how to pass the input to the model.

Some examples simply concatenate prompt and target and pass them via `input_ids`. But how are you supposed to tell the model where the prompt ends and the target begins?

The 2 most intuitive candidates from reading the docs seem to be `token_type_ids` and `labels`, but …

deeplearning format gpt huggingface seq2seq