Initial Representation for Concatenated [x, y] Sequences

A data scientist is using a large language model to determine the conditional log-probability of a specific completion y following a given prompt x. Their process involves concatenating the two sequences into [x, y] and then performing a single forward pass to compute the log-probability of this combined sequence, which they take as their final result. Which statement best analyzes the flaw in this methodology?

You are tasked with using a large language model to compute the conditional log-probability of an output sequence y given an input sequence x. Arrange the following computational steps into the correct chronological order.

Calculating Conditional Log-Probability from Model Outputs