input: Write a function to generate a random tic tac toe board.
  assert isinstance(generate_board(), list)
  assert len(generate_board()) == 3
  assert all(len(row) == 3 for row in generate_board())
output: import random
  
  def generate_board():
      return [[random.choice(['X', 'O', '']) for _ in range(3)] for _ in range(3)]