KEP-2170: Add PyTorch DDP MNIST training example #1942

Job	Run time
integration-test (v1.30.6, none, 3.10)	19m 44s
integration-test (v1.29.2, none, 3.10)	19m 47s
integration-test (v1.28.7, none, 3.8)	20m 6s
integration-test (v1.30.6, scheduler-plugins, 3.10)	19m 59s
integration-test (v1.29.2, scheduler-plugins, 3.9)	19m 3s
integration-test (v1.28.7, scheduler-plugins, 3.11)	19m 6s
integration-test (v1.30.6, volcano, 3.10)	22m 30s
integration-test (v1.29.2, volcano, 3.8)	19m 49s
integration-test (v1.28.7, volcano, 3.10)	18m 42s
	2h 58m 46s

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