input: Write a function to generate a random 6-digit number in Python. The function should return the generated number.
  assert generate_6_digit_number()>99999
  assert generate_6_digit_number()<1000000
  assert len(str(generate_6_digit_number()))==6
  assert isinstance(generate_6_digit_number(), int)
output: import random
  
  def generate_6_digit_number():
      return random.randint(100000, 999999)