[doc] Fix to DDP tutorial #3236

Job	Run time
pytorch_tutorial_build_worker (1, 15, linux.g5.12xlarge.nvidia.gpu)	14m 22s
pytorch_tutorial_build_worker (2, 15, linux.g5.4xlarge.nvidia.gpu)	49m 59s
pytorch_tutorial_build_worker (3, 15, linux.g5.4xlarge.nvidia.gpu)	18m 55s
pytorch_tutorial_build_worker (4, 15, linux.g5.4xlarge.nvidia.gpu)	27m 25s
pytorch_tutorial_build_worker (5, 15, linux.g5.4xlarge.nvidia.gpu)	19m 46s
pytorch_tutorial_build_worker (6, 15, linux.g5.4xlarge.nvidia.gpu)	21m 49s
pytorch_tutorial_build_worker (7, 15, linux.4xlarge.nvidia.gpu)	32m 48s
pytorch_tutorial_build_worker (8, 15, linux.4xlarge.nvidia.gpu)	20m 34s
pytorch_tutorial_build_worker (9, 15, linux.4xlarge.nvidia.gpu)	33m 54s
pytorch_tutorial_build_worker (10, 15, linux.4xlarge.nvidia.gpu)	13m 49s
pytorch_tutorial_build_worker (11, 15, linux.4xlarge.nvidia.gpu)	12m 21s
pytorch_tutorial_build_worker (12, 15, linux.4xlarge.nvidia.gpu)	18m 52s
pytorch_tutorial_build_worker (13, 15, linux.4xlarge.nvidia.gpu)	17m 35s
pytorch_tutorial_build_worker (14, 15, linux.4xlarge.nvidia.gpu)	12m 53s
pytorch_tutorial_build_worker (15, 15, linux.4xlarge.nvidia.gpu)	26m 11s
pytorch_tutorial_build_manager	20m 24s
	6h 1m 37s

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