input: Write a function to take a positive integer as input and use a while loop to output the numbers counting down to 1. If the input is not a positive integer, the function should raise a ValueError.
  
  def count_down(num):
      if num <= 0:
          raise ValueError("Input must be a positive integer")
      while num > 0:
          print(num)
          num -= 1
  
  assert count_down(6)
  assert count_down(1)
  assert count_down(0)  # Should raise ValueError
  assert count_down(-1)  # Should raise ValueError
output: def count_down(num):
      if num <= 0:
          raise ValueError("Input must be a positive integer")
      while num > 0:
          print(num)
          num -= 1