input: Write a function to count down from a given positive integer to 1 using a while loop and print each number. If the input is not a positive integer, the function should print "Invalid input".
  
  assert count_down(6) == None
  assert count_down(3) == None
  assert count_down(0) == None
  assert count_down(-5) == None
  
  Explanation:
  - The first assert checks if the function correctly counts down from 6 to 1.
  - The second assert checks if the function correctly counts down from 3 to 1.
  - The third assert checks if the function handles the input of 0 correctly by printing "Invalid input".
  - The fourth assert checks if the function handles negative input correctly by printing "Invalid input".
output: def count_down(n):
      if n <= 0:
          print("Invalid input")
      else:
          while n > 0:
              print(n)
              n -= 1