input: Write a function to convert ratings in a list of strings to a float. The function should return a list of these converted float values. 
  assert str_to_float(['4.5', '3.0', '2.5', '4.0'])==[4.5, 3.0, 2.5, 4.0]
  assert str_to_float(['0.0', '1.1', '2.2'])==[0.0, 1.1, 2.2]
  assert str_to_float([])==[]
output: def str_to_float(ratings):
      return [float(rating) for rating in ratings]