“The Wall,” a major retrospective of Chinese contemporary art from the last 20 years, is a thought-provoking exhibition that explores the many walls – physical and metaphorical – that unite and divide Chinese society. The show, which runs here until Sunday and opens at the State University at Buffalo on Oct. 1, includes installations, performance art, video, photography and painting. These are displayed, fittingly enough, in the maze of circling walls and hidden cul-de-sacs that make up the exhibition space of the Millennium Monument Art Museum.