Amma, that's the sweet little restaurant that opened recently on East 51st Street.The pale mustard walls and salmon-coral banquettes are inviting, an improvement on the usual brass, dark wood and beads. And though the menu is largely familiar, Amma, taking a cue from Indian-fusion restaurants like Tamarind, serves main courses on individual plates, with nan, rice and vegetables, rather than family style.The service is genial, and the rosewater lassi actually tastes and smells of rosewater.