input: Write a function to generate a 7x7 pattern of '#' and ' ' characters based on the sum of the row and column indices. If the sum is even, print a '#'. If the sum is odd, print a ' '. The function should return None.
  assert generate_pattern()==None
  for i in range(7):
      for j in range(7):
          if (i + j) % 2 == 0:
              print('#', end='')
          else:
              print(' ', end='')
      print()
output: def generate_pattern():
      for i in range(7):
          for j in range(7):
              if (i + j) % 2 == 0:
                  print('#', end='')
              else:
                  print(' ', end='')
          print()