Add download instructions for pretrained model in dynamic quantization tutorial #4470

Job	Run time
pytorch_tutorial_build_worker (5, 15, linux.g5.4xlarge.nvidia.gpu)	23m 24s
pytorch_tutorial_build_worker (1, 15, linux.g5.12xlarge.nvidia.gpu)	15m 20s
pytorch_tutorial_build_worker (3, 15, linux.g5.4xlarge.nvidia.gpu)	21m 45s
pytorch_tutorial_build_worker (4, 15, linux.g5.4xlarge.nvidia.gpu)	35m 29s
pytorch_tutorial_build_worker (7, 15, linux.g5.4xlarge.nvidia.gpu)	19m 23s
pytorch_tutorial_build_worker (9, 15, linux.g5.4xlarge.nvidia.gpu)	17m 48s
pytorch_tutorial_build_worker (8, 15, linux.g5.4xlarge.nvidia.gpu)	23m 52s
pytorch_tutorial_build_worker (10, 15, linux.g5.4xlarge.nvidia.gpu)	16m 37s
pytorch_tutorial_build_worker (6, 15, linux.g5.4xlarge.nvidia.gpu)	22m 7s
pytorch_tutorial_build_worker (2, 15, linux.g5.4xlarge.nvidia.gpu)	39m 59s
pytorch_tutorial_build_worker (12, 15, linux.g5.4xlarge.nvidia.gpu)	20m 36s
pytorch_tutorial_build_worker (11, 15, linux.g5.4xlarge.nvidia.gpu)	15m 18s
pytorch_tutorial_build_worker (15, 15, linux.g5.4xlarge.nvidia.gpu)	31m 22s
pytorch_tutorial_build_worker (13, 15, linux.g5.4xlarge.nvidia.gpu)	28m 30s
pytorch_tutorial_build_worker (14, 15, linux.g5.4xlarge.nvidia.gpu)	26m 17s
pytorch_tutorial_build_manager	22m 58s
	6h 20m 45s

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