input: Write a python function to generate a random string of length 5 consisting of lowercase letters and digits.
  assert len(generate_random_string()) == 5
  assert all(c.isalnum() and c.islower() for c in generate_random_string())
  assert generate_random_string() != generate_random_string()
output: import random
  import string
  
  def generate_random_string():
      return ''.join(random.choices(string.ascii_lowercase + string.digits, k=5))