[feature request] Add CPU example

[gaocegege]

Description

Add CPU ImageNet or MNIST example

Motivation/Background

Now we only have GPU ImageNet example, which requires multiple GPUs to test the feature. It is better to support CPU.

I hacked the GPU ImageNet example to support CPU locally. I'd appreciate if we can support it in this repo.

[andrewemendez]

How do you support imageNet to support CPU locally, would love to learn more and setup that myself!

[gaocegege]

https://github.com/kubeflow/training-operator/blob/master/examples/pytorch/elastic/imagenet/imagenet.py