As I recall, Gerald Ford pardoned Nixon for any federal crimes he might have committed as President while in office. If New York AG wants to get publicity for prosecuting Trump there is no pardon that can protect him. However, a Biden DOJ prosecution of his predecessor would set a precedent that Joe would never set... as he himself might be the object of revenge some day.
Link: https://caselaw.findlaw.com/us-supreme-court/71/333.html