input: Write a function to generate a unique 6 digit number.
  assert generate_number()>=10000 and generate_number()<=99999
  assert generate_number()!=generate_number()
  assert len(str(generate_number()))==6
  assert isinstance(generate_number(), int)
  assert generate_number()%1==0
output: import random
  
  def generate_number():
      return random.randint(100000, 999999)