HE is described as an ''alto-relief'' sculptor. Although it is said that to get relief from Bill Mack's sales patter which descends like the rattle of machine-gun fire is another thing. An exhibition of the American sculptor's work opened yesterday at the Vincent Lee Fine Arts Gallery, situated on the ground floor of the Ritz Carlton. Ten pieces of alto-relief art - which can be loosely defined as Romanesque two-dimensional wall sculpture - are on display. Mack, who dedicates his time and talent to raise funds for AIDS Research, is a devoted family man. So much so that the figurative expressions of his sculptures are courtesy of his wife Deborah. He explains: ''The process is so time-consuming that only a willing and dedicated partner has the patience for the long modelling sessions.''