MORE than 50 years after they were executed as So viet spies, the case of Julius and Ethel Rosenberg continues to generate intense emotion on both sides of the political spectrum. For the left, it has long been an article of faith that two wholly innocent radicals were framed and executed solely because of their politics. Now, the couple's granddaughter, Ivy Meeropol, has made a documentary that largely reinforces that viewpoint. Heir to an Execution, which premieres Monday on HBO, is a portrait of two people so filled with love and warmth they couldn't possibly have betrayed their country.