Fixed links in process_group_cpp_extension_tutorial.rst (#3157) #3432

Job	Run time
pytorch_tutorial_build_worker (1, 15, linux.g5.12xlarge.nvidia.gpu)	11m 11s
pytorch_tutorial_build_worker (2, 15, linux.g5.4xlarge.nvidia.gpu)	48m 55s
pytorch_tutorial_build_worker (3, 15, linux.g5.4xlarge.nvidia.gpu)	17m 15s
pytorch_tutorial_build_worker (4, 15, linux.g5.4xlarge.nvidia.gpu)	29m 24s
pytorch_tutorial_build_worker (5, 15, linux.g5.4xlarge.nvidia.gpu)	18m 19s
pytorch_tutorial_build_worker (6, 15, linux.g5.4xlarge.nvidia.gpu)	15m 46s
pytorch_tutorial_build_worker (7, 15, linux.4xlarge.nvidia.gpu)	17m 23s
pytorch_tutorial_build_worker (8, 15, linux.4xlarge.nvidia.gpu)	11m 33s
pytorch_tutorial_build_worker (9, 15, linux.4xlarge.nvidia.gpu)	17m 55s
pytorch_tutorial_build_worker (10, 15, linux.4xlarge.nvidia.gpu)	14m 10s
pytorch_tutorial_build_worker (11, 15, linux.4xlarge.nvidia.gpu)	13m 52s
pytorch_tutorial_build_worker (12, 15, linux.4xlarge.nvidia.gpu)	28m 29s
pytorch_tutorial_build_worker (13, 15, linux.4xlarge.nvidia.gpu)	36m 11s
pytorch_tutorial_build_worker (14, 15, linux.4xlarge.nvidia.gpu)	30m 32s
pytorch_tutorial_build_worker (15, 15, linux.4xlarge.nvidia.gpu)	24m 56s
pytorch_tutorial_build_manager	17m 43s
	5h 53m 34s

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