input: Write a python function to initialize a BOARD_SIZE x BOARD_SIZE board filled with zeros.
  assert (initialize_board(4) == np.zeros((4, 4))).all()
  assert (initialize_board(3) == np.zeros((3, 3))).all()
  assert (initialize_board(2) == np.zeros((2, 2))).all()
output: def initialize_board(BOARD_SIZE):
      import numpy as np
      return np.zeros((BOARD_SIZE, BOARD_SIZE))