[doc] ddp multigpu tutorial - small updates #3343

Job	Run time
pytorch_tutorial_build_worker (1, 15, linux.g5.12xlarge.nvidia.gpu)	11m 16s
pytorch_tutorial_build_worker (2, 15, linux.g5.4xlarge.nvidia.gpu)	48m 29s
pytorch_tutorial_build_worker (3, 15, linux.g5.4xlarge.nvidia.gpu)	17m 11s
pytorch_tutorial_build_worker (4, 15, linux.g5.4xlarge.nvidia.gpu)	27m 10s
pytorch_tutorial_build_worker (5, 15, linux.g5.4xlarge.nvidia.gpu)	18m 25s
pytorch_tutorial_build_worker (6, 15, linux.g5.4xlarge.nvidia.gpu)	15m 12s
pytorch_tutorial_build_worker (7, 15, linux.4xlarge.nvidia.gpu)	18m 49s
pytorch_tutorial_build_worker (8, 15, linux.4xlarge.nvidia.gpu)	14m 9s
pytorch_tutorial_build_worker (9, 15, linux.4xlarge.nvidia.gpu)	17m 52s
pytorch_tutorial_build_worker (10, 15, linux.4xlarge.nvidia.gpu)	15m 12s
pytorch_tutorial_build_worker (11, 15, linux.4xlarge.nvidia.gpu)	25m 28s
pytorch_tutorial_build_worker (12, 15, linux.4xlarge.nvidia.gpu)	31m 59s
pytorch_tutorial_build_worker (13, 15, linux.4xlarge.nvidia.gpu)	38m 1s
pytorch_tutorial_build_worker (14, 15, linux.4xlarge.nvidia.gpu)	23m 17s
pytorch_tutorial_build_worker (15, 15, linux.4xlarge.nvidia.gpu)	16m 55s
pytorch_tutorial_build_manager	18m 32s
	5h 57m 57s

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