input: Write a function to generate a random number between the given input numbers start and stop (both inclusive). If either the start or stop is not provided, the function should generate a random number between 0 and 10.
  assert random_num(1, 5) in range(1, 6)
  assert random_num(-1, 1) in range(-1, 2)
  assert random_num(0, 0)==0
  assert random_num() in range(0, 11)
  assert random_num(10) in range(0, 11)
output: import random
  
  def random_num(start=None, stop=None):
      if start is None:
          start = 0
      if stop is None:
          stop = 10
      return random.randint(start, stop)