The Film Forum in New York is continuing its noble experiment in offbeat programming. Through Nov. 10, it is presenting the American theatrical premiere of Lorang's Way together with Under the Men's Tree, both anthropological documentaries by David and Judith MacDougall. Another documentary was also on the screen - September Wheat, a visually inventive documentary on world hunger by West German director Peter Krieg, paired with a 1909 short by D. W. Griffith. And more recently, in an unusual booking, the Film Forum presented a rare engagement of Tiefland by Leni Riefenstahl, the former Hitler propagandist whose works are praised by some film scholars for a formal ingenuity that's as aesthetically appealing as her politics have sometimes been loathsome. ''Tiefland'' is no fascist documentary like ''The Triumph of the Will,'' though its visual style reflects Riefenstahl's worship of physical beauty and simplistic view of moral issues. The filmmaker/star looks lovely, however - the movie was begun in 1942 but not finished until 1954 - and the story about a wicked landlord, a beautiful woman, and a simple but courageous peasant is nothing if not atmospheric. An interesting oddity from the movie world. Coming up next, more documentaries: Garlic is as Good as Ten Mothers and Werner Herzog Eats His Shoe, by Les Blank, and Stoney Knows How, by Alan Govenar and Bruce Lane.