Elizabeth Taylor, the 12-year-old actress who made her screen debut in “Lassie Come Home” and hat the lead opposite Mickey Rooney in “National Velvet/* will be starred by Metro in “Hold High the Torch.** It’s the story of a child and a war dog, the third of the Lassie pictures. The role was originally meant for Margaret O’Brien, but Elizabeth has proved talented enough to inherit it. JL_