input: Write a function to generate a random string of a given length using lowercase letters.
  assert generate_random_string(5) != generate_random_string(5)
  assert len(generate_random_string(10)) == 10
  assert all(c in string.ascii_lowercase for c in generate_random_string(7))
output: import random
  import string
  
  def generate_random_string(length):
      return ''.join(random.choice(string.ascii_lowercase) for _ in range(length))