docs: Fix incorrect usage description of ctx.save_for_backward #4482

Job	Run time
pytorch_tutorial_build_worker (4, 15, linux.g5.4xlarge.nvidia.gpu)	38m 10s
pytorch_tutorial_build_worker (3, 15, linux.g5.4xlarge.nvidia.gpu)	23m 7s
pytorch_tutorial_build_worker (5, 15, linux.g5.4xlarge.nvidia.gpu)	24m 0s
pytorch_tutorial_build_worker (6, 15, linux.g5.4xlarge.nvidia.gpu)	22m 55s
pytorch_tutorial_build_worker (7, 15, linux.g5.4xlarge.nvidia.gpu)	20m 13s
pytorch_tutorial_build_worker (8, 15, linux.g5.4xlarge.nvidia.gpu)	22m 57s
pytorch_tutorial_build_worker (2, 15, linux.g5.4xlarge.nvidia.gpu)	40m 51s
pytorch_tutorial_build_worker (9, 15, linux.g5.4xlarge.nvidia.gpu)	18m 42s
pytorch_tutorial_build_worker (12, 15, linux.g5.4xlarge.nvidia.gpu)	21m 25s
pytorch_tutorial_build_worker (11, 15, linux.g5.4xlarge.nvidia.gpu)	16m 12s
pytorch_tutorial_build_worker (1, 15, linux.g5.12xlarge.nvidia.gpu)	15m 53s
pytorch_tutorial_build_worker (10, 15, linux.g5.4xlarge.nvidia.gpu)	17m 20s
pytorch_tutorial_build_worker (13, 15, linux.g5.4xlarge.nvidia.gpu)	28m 29s
pytorch_tutorial_build_worker (15, 15, linux.g5.4xlarge.nvidia.gpu)	30m 53s
pytorch_tutorial_build_worker (14, 15, linux.g5.4xlarge.nvidia.gpu)	27m 8s
pytorch_tutorial_build_manager	23m 10s
	6h 31m 25s

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