[10 pts] Read the ratings file to return a user-to-movies dictionary that maps user ID to the associated movies and the corresponding ratings. Write a function named read_user_ratings for this, with the ratings file as the parameter. For example: { u1: [ (m1, r1), (m2, r2) ], u2: [ (m3, r3), (m8, r8) ] } where ui is user ID, mi is movie, ri is corresponding rating.