input: Write a python function to initialize a game board with the given number of rows and columns.
  assert isinstance(Board(3, 3).grid, list) == True
  assert len(Board(3, 3).grid) == 3
  assert all(len(row) == 3 for row in Board(3, 3).grid) == True
output: class Board:
      def __init__(self, rows, cols):
          self.grid = [[None for _ in range(cols)] for _ in range(rows)]