Parallelize neuron training processes for each neuron core #464

Job	Run time
build-image-neuron-inference	4m 25s
build-image-neuron-training	6m 40s
build-image	6m 43s
build	2m 52s
build-image-neuronx	14m 33s
build-image-nvidia	4m 40s
build-image-nvidia-inference	7m 15s
build-image-nvidia-training	9m 5s
build-test	1m 17s
	57m 30s

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