input: Write a function to generate a unique 6 digit number. The function should return a randomly generated number between 100000 and 999999. If the generated number is already in use, the function should generate a new number until a unique one is found.
  assert generate_number() != generate_number()
  assert 100000 <= generate_number() <= 999999
output: 
  import random
  
  def generate_number():
      used_numbers = set()
      while True:
          number = random.randint(100000, 999999)
          if number not in used_numbers:
              used_numbers.add(number)
              return number
  