LOS ANGELS: Natalie Portman says motherhood is the hardest job in the world.  The 'Thor: The Dark World' actress claims raising her two-year-old son Aleph with her husband, French choreographer Benjamin Millepied, is far more demanding and exhausting than any acting role she has ever undertaken.  She said: ''The one thing my mom didn't warn me about is how hard it is to be a mother yourself. I never thought of motherhood as being the full-on job that it is. Going to work now feels like a day off.'' Portman believes her special bond with her parents is responsible for being a sensible, ''conservative'' person.–ES