Working against a tight deadline, at least 20,000 Japanese police fanned out today in a desperate search for cult leaders suspected in last month'snerve gas attacks on the Tokyo subway. Coast guard patrols checked boats leaving the country, and police in Tokyo went on "emergency alert." The reason: in a book released last month,Aum Shinri Kyo cultleader Shoko Asahara predicted a disaster in Tokyo this weekend. Other police units concentrated on crowded neighborhoods in case cult members interpreted the prediction as a sign to do damage. At the same time, other cult leaders went on trial in Moscow for "corrupting" the 35,000 young followers it claims in Russia. The plaintiffs are Russian parents.