input: Write a function to convert a given string to a list of integers.
  def to_int_list(s):
      return [int(x) for x in s.split(',')]
  assert to_int_list("1,2,3,4")==[1,2,3,4]
  assert to_int_list("5,6,7,8")==[5,6,7,8]
  assert to_int_list("10,20,30,40")==[10,20,30,40]
  assert to_int_list("100,200,300,400")==[100,200,300,400]
  assert to_int_list("1000,2000,3000")==[1000,2000,3000]
output: def to_int_list(s):
      return [int(x) for x in s.split(',')]