input: Write a python function to print a grid of given size, where each cell contains the size of the grid.
  assert print_grid(3) == "3 3 3 \n3 3 3 \n3 3 3 \n"
  assert print_grid(2) == "2 2 \n2 2 \n"
  assert print_grid(1) == "1 \n"
output: def print_grid(size):
      return '\n'.join([' '.join([str(size)] * size) for _ in range(size)]) + '\n'