fix a typo in zeroing_out_gradients.py #4260

Job	Run time
pytorch_tutorial_build_worker (6, 15, linux.g5.4xlarge.nvidia.gpu)	22m 55s
pytorch_tutorial_build_worker (2, 15, linux.g5.4xlarge.nvidia.gpu)	33m 44s
pytorch_tutorial_build_worker (4, 15, linux.g5.4xlarge.nvidia.gpu)	36m 41s
pytorch_tutorial_build_worker (7, 15, linux.g5.4xlarge.nvidia.gpu)	25m 46s
pytorch_tutorial_build_worker (10, 15, linux.g5.4xlarge.nvidia.gpu)	17m 43s
pytorch_tutorial_build_worker (1, 15, linux.g5.12xlarge.nvidia.gpu)	13m 30s
pytorch_tutorial_build_worker (9, 15, linux.g5.4xlarge.nvidia.gpu)	18m 54s
pytorch_tutorial_build_worker (8, 15, linux.g5.4xlarge.nvidia.gpu)	15m 53s
pytorch_tutorial_build_worker (12, 15, linux.g5.4xlarge.nvidia.gpu)	20m 51s
pytorch_tutorial_build_worker (11, 15, linux.g5.4xlarge.nvidia.gpu)	16m 45s
pytorch_tutorial_build_worker (5, 15, linux.g5.4xlarge.nvidia.gpu)	24m 6s
pytorch_tutorial_build_worker (14, 15, linux.g5.4xlarge.nvidia.gpu)	24m 50s
pytorch_tutorial_build_worker (3, 15, linux.g5.4xlarge.nvidia.gpu)	23m 6s
pytorch_tutorial_build_worker (15, 15, linux.g5.4xlarge.nvidia.gpu)	29m 42s
pytorch_tutorial_build_worker (13, 15, linux.g5.4xlarge.nvidia.gpu)	30m 11s
pytorch_tutorial_build_manager	26m 9s
	6h 20m 46s

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