input: Write a function to print a 7x7 grid where each cell has a '#' if the sum of the row index and column index is even, and a space if it is odd.
  assert checkerboard(7, 7)==None
  Input: checkerboard(7, 7)
  Output: 
  
  # # # # # 
   # # # # #
  # # # # # 
   # # # # #
  # # # # # 
   # # # # #
  # # # # # 
  
output: def checkerboard(rows, cols):
      for i in range(rows):
          for j in range(cols):
              if (i + j) % 2 == 0:
                  print('#', end=' ')
              else:
                  print(' ', end=' ')
          print()