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()=="A1B2C3D4"
  assert generate_random_string()=="X5Y6Z7W8"
  assert generate_random_string()=="9Q8W7E6R"
  
output: import random
  import string
  
  def generate_random_string():
      return ''.join(random.choices(string.ascii_uppercase + string.digits, k=8))