input: Write a function to generate a random string of length 8 made of uppercase letters and numbers. The function should take no arguments and return the generated string.
  assert generate_random_string()=="X8Z9K2L4"
  assert generate_random_string()=="A3B7C6D5"
  assert generate_random_string()!="A1B2C3D4"
  assert len(generate_random_string())==8
output: import random
  import string
  
  def generate_random_string():
      return ''.join(random.choices(string.ascii_uppercase + string.digits, k=8))