Why Japanese cruisers carried heavy torpedo batteries

Did Japanese doctrine call for cruisers to fire their torpedoes as soon as a gunfight began?

Generally yes: long-range torpedoes were meant to be launched early, especially in night attacks, before close gunfire exposed the ship and its tubes.

Japanese doctrine called for long-range torpedoes to be launched as an action began. In a decisive-battle setting, night torpedo attacks could reduce an enemy before the main engagement. Early launch also kept a cruiser from carrying its full offensive load deep into a close gunfight. The doctrine explains the tactical attraction of the tubes and the catastrophic danger of fuelled, loaded torpedoes once a cruiser came under fire.