Sweater sets are a hit for spring. At some of the top design houses, they even outrank suits. And contrary to popular belief, they're not too hot to handle. For chilly spring evenings in Los Angeles, nothing is better than the cozy comfort of lightweight cashmere knit. And for Southern California's high-mercury afternoons, the coolest cut of all could be a matched pair of silk, cotton or linen sweaters. The most capricious idea of the season is the sleeveless, turtleneck style that turns up in many New York collections. Why bare the arms and shoulders but swaddle the neck in knit? Because it happens to look great, the sweater savants say. At right, Bill Blass' idea of the spring sweater set. It's red cashmere, lined in stripes, collared and cuffed in white to match the pants. At top, far right, Ralph Lauren's black sweater set shown with blue, straight-leg pants. Calvin Klein's white sleeveless turtleneck and cardigan sweaters are at top, near right, also shown with white pants. And at left, Louis Dell'Olio's slinky, silvery evening pants topped by gray knit sweater and coat, for Anne Klein .