input: Write a function to generate a random string of length 8 made of uppercase letters and numbers. The function should return the generated string.
  assert generate_random_string()=='A3B8E6K9'
  assert generate_random_string()=='7R2VX5Z1'
  assert generate_random_string()=='L4N7T9W3'
  assert generate_random_string()=='5P6Q8S1S'
  assert generate_random_string()=='J2K5M8N4'
output: import random
  import string
  
  def generate_random_string():
      return ''.join(random.choices(string.ascii_uppercase + string.digits, k=8))