input: Write a function to generate a 7x7 grid of characters where each character is '#' if the sum of its row and column indices is even, and a space if the sum is odd. The function should print the grid as its output.
  def print_grid():
      for i in range(7):
          for j in range(7):
              if (i + j) % 2 == 0:
                  print('#', end='')
              else:
                  print(' ', end='')
          print()
  
  assert print_grid()==None
  assert print_grid()==None
  assert print_grid()==None
  assert print_grid()==None
  assert print_grid()==None
output: def print_grid():
      for i in range(7):
          for j in range(7):
              if (i + j) % 2 == 0:
                  print('#', end='')
              else:
                  print(' ', end='')
          print()