WITH HEADS HUNG LOW Some players speaking in barely a whisper, the Giants walked into their locker room at FedEx Field after beating the Washington Redskins on Sunday, acting nothing like a team that had just won its 10th game of the season.They had just bounced back from an embarrassing two-game slide to defeat Washington, 17-14. Yet there was not a celebration or a smile in sight. Because their season, which just two weeks ago seemed so filled with promise, had come to a painful end.