[doc] Fix to DDP tutorial #3231

Job	Run time
pytorch_tutorial_build_worker (1, 15, linux.g5.12xlarge.nvidia.gpu)	8m 10s
pytorch_tutorial_build_worker (2, 15, linux.g5.4xlarge.nvidia.gpu)	5m 23s
pytorch_tutorial_build_worker (3, 15, linux.g5.4xlarge.nvidia.gpu)	5m 16s
pytorch_tutorial_build_worker (4, 15, linux.g5.4xlarge.nvidia.gpu)	5m 19s
pytorch_tutorial_build_worker (5, 15, linux.g5.4xlarge.nvidia.gpu)	5m 18s
pytorch_tutorial_build_worker (6, 15, linux.g5.4xlarge.nvidia.gpu)	5m 18s
pytorch_tutorial_build_worker (7, 15, linux.4xlarge.nvidia.gpu)	8m 18s
pytorch_tutorial_build_worker (8, 15, linux.4xlarge.nvidia.gpu)	8m 18s
pytorch_tutorial_build_worker (9, 15, linux.4xlarge.nvidia.gpu)	8m 18s
pytorch_tutorial_build_worker (10, 15, linux.4xlarge.nvidia.gpu)	8m 19s
pytorch_tutorial_build_worker (11, 15, linux.4xlarge.nvidia.gpu)	8m 19s
pytorch_tutorial_build_worker (12, 15, linux.4xlarge.nvidia.gpu)	8m 20s
pytorch_tutorial_build_worker (13, 15, linux.4xlarge.nvidia.gpu)	8m 18s
pytorch_tutorial_build_worker (14, 15, linux.4xlarge.nvidia.gpu)	8m 18s
pytorch_tutorial_build_worker (15, 15, linux.4xlarge.nvidia.gpu)	8m 7s
pytorch_tutorial_build_manager	1s
	1h 49m 20s

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