Umasrija Sarkar knows firsthand how music and art can bring people together. “Ever since I was little growing up, I’ve always loved just giving back to people. Our family was really big on that,” Sarkar said. Sarkar, a junior at Belmont High School, is organizing “Rhythm and Strings,” a benefit concert to support the Watertown Food Pantry and Belmont Helps. The performances range from a viola ensemble — which Sarkar will be a part of — to an Indian dance.…