Is one held accountable for visiting the Prophet's Mosque with ablution (wudu') and then going to Al-Baqi' without ablution?

There is no obligation on one who visits Al-Baqi' or the martyrs of Uhud or all graves while not in a state of ablution, because ritual purity is not a condition for visiting graves. Visiting graves is recommended due to the general saying of the Prophet, peace and blessings be upon him: "Visit graves, for they remind you of the Hereafter."