What's the eval_pred of function compute_metrics when using GPT2LMHeadModel?

I am using trainer to train a GPT2LMHeadModel, this is my function.

I wonder to get the ‘loss’ calculated by the model, however it tell me that I can’t get what I want.
I also find the format of eval_pred as below

But I can’t figure out what is in the eval_pred.predictions. Maybe someone can help me and tell me what it is?