KEP-2170: Add PyTorch DDP MNIST training example #1936

Job	Run time
integration-test (v1.30.6, none, 3.10)	20m 53s
integration-test (v1.29.2, none, 3.10)	19m 19s
integration-test (v1.28.7, none, 3.8)	19m 33s
integration-test (v1.30.6, scheduler-plugins, 3.10)	19m 43s
integration-test (v1.29.2, scheduler-plugins, 3.9)	19m 17s
integration-test (v1.28.7, scheduler-plugins, 3.11)	19m 18s
integration-test (v1.30.6, volcano, 3.10)	18m 32s
integration-test (v1.29.2, volcano, 3.8)	19m 10s
integration-test (v1.28.7, volcano, 3.10)	19m 12s
	2h 54m 57s

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