code: def prefetch_users(persistent_course_grades):
          
          users = User.objects.filter(
              id__in=[grade.user_id for grade in persistent_course_grades]
          )
          return {
              user.id: user for user in users
          }
docs: Prefetch Users from the list of user_ids present in the persistent_course_grades.
  
          Arguments:
              persistent_course_grades (list): A list of PersistentCourseGrade.
  
          Returns:
              (dict): A dictionary containing user_id to user mapping.