input: Write a function to generate a Python program that takes two numbers as input, adds them, and then doubles the result. If the input is not provided, the function should return 0.
  
  def double_add(num1=0, num2=0):
      return (num1 + num2) * 2
  
  assert double_add(2, 3) == 10
  assert double_add(5, 5) == 20
  assert double_add(0, 0) == 0
  assert double_add(-1, 1) == 0
  assert double_add() == 0
output: def double_add(num1=0, num2=0):
      return (num1 + num2) * 2