SUMO SOON - In case you haven't witnessed sumo, the Japanese version of grunt and groan, you'll nave your chance for two weeks beginning about the middle of May . . .  The matches, sponsored by the Wrestlers' Association and held in Tokyo at the Kokugi-kan, the amphitheater at Kuramae near the 8167th Army Hospital, are worth seeing once . . . The wrestlers, all of whom tip the scales at around 300 pounds, warm up and hold their prelims in the morning. The main bouts are held in the afternoon following a ceremony called dohyo-iri, in which the current champion is the central figure attended by some 300 champions and hopefuls. . . Although there are many rules, the basic idea of sumo is to force an opponent to touch the mat with any part of his body other than the soles of his feet, thus costing him the match. Make reservations - it's a popular sport.