input: Write a function to generate a unique 6 digit number. The function should return the generated number.
  assert generate_number()==12345
  assert generate_number()==67890
  assert generate_number()==54321
  assert generate_number()==98765
  assert generate_number()==11223
  
  ### Explanation:
  - The function `generate_number` should generate a unique 6-digit number each time it is called.
  - The assert statements check if the function returns different 6-digit numbers.
  - The specific numbers used in the asserts are examples and should be replaced with actual generated numbers during testing.
output: import random
  
  def generate_number():
      return random.randint(100000, 999999)