input: Write a function to generate a unique 6 digit number. If the function is called multiple times, it should always return a different number. If the number generation is successful, return the generated number, otherwise return None.
  assert generate_number()!=generate_number()
  assert generate_number() >= 10000
  assert generate_number() <= 99999
  assert generate_number() != None
output: 
  import random
  
  def generate_number():
      return random.randint(100000, 999999)
  