input: Write a python function to control pyautogui based on input commands.
  assert control_pyautogui('go forward')==None
  assert control_pyautogui('turn left')==None
  assert control_pyautogui('turn right')==None
  assert control_pyautogui('stop')==None
  assert control_pyautogui('look up')==None
  assert control_pyautogui('invalid command')=='Invalid command.'
output: import pyautogui
  
  def control_pyautogui(command):
      if command == 'go forward':
          pyautogui.keyDown('w')
          pyautogui.keyUp('w')
      elif command == 'turn left':
          pyautogui.keyDown('a')
          pyautogui.keyUp('a')
      elif command == 'turn right':
          pyautogui.keyDown('d')
          pyautogui.keyUp('d')
      elif command == 'stop':
          pyautogui.keyDown('s')
          pyautogui.keyUp('s')
      elif command == 'look up':
          pyautogui.keyDown('up')
          pyautogui.keyUp('up')
      else:
          return 'Invalid command.'