Session Step Duration Analysis

You are analyzing user behavior in a multi-step workflow system and need to calculate how long users typically spend on each step. Given a list of session records where each record is formatted as `[session_id, step, timestamp]`, you need to calculate the average time consumed per step across all sessions. Each session progresses through numbered steps (1, 2, 3, ...), and the time consumed at a step is the difference between that step's timestamp and the previous step's timestamp within the same session. Return a dictionary mapping each step number to the average time consumed at that step.