input: Write a python function to assign a grade based on given marks.
  assert assign_grade(95) == 'A'
  assert assign_grade(85) == 'B'
  assert assign_grade(75) == 'C'
  assert assign_grade(65) == 'D'
  assert assign_grade(55) == 'F'
output: def assign_grade(marks):
      if marks >= 90:
          return 'A'
      elif marks >= 80:
          return 'B'
      elif marks >= 70:
          return 'C'
      elif marks >= 60:
          return 'D'
      else:
          return 'F'