input: Write a function to print the absolute difference between two given numbers. If no numbers are provided, the function should return 0.
  assert absolute_difference(9, 5)==4
  assert absolute_difference(0, 0)==0
  assert absolute_difference(-2, 3)==5
  assert absolute_difference(10, -10)==20
  assert absolute_difference()==0
output: def absolute_difference(a=0, b=0):
      return abs(a - b)