Parallelize neuron training processes for each neuron core #470

Job	Run time
build	2m 53s
build-image-neuron-inference	4m 41s
build-image	5m 52s
build-image-neuron-training	6m 30s
build-image-neuronx	14m 9s
build-image-nvidia	4m 37s
build-image-nvidia-inference	7m 55s
build-image-nvidia-training	9m 17s
build-test	1m 22s
	57m 16s

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