Replace kubeflow-images-staging with kubeflow-images-public

[ankushagarwal]

Related to kubeflow/kubeflow#534

/cc elsonrodriguez
/cc jose5918

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This change is 

[gaocegege]

/lgtm

[k8s-ci-robot]

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[jose5918]

@gaocegege @ankushagarwal The logs for this PR don't seem to have pushed the container as intended Pushing gcr.io/kubeflow-ci/pytorch-operator:7182db7

Edit: Maybe I'm confused about the purpose of this PR because I don't know if that parameter was in use

[ankushagarwal]

@jose5918 : I'm not sure if I follow your comment. The purpose of this PR was to move from kubeflow-images-staging to kubeflow-images-public for hosting the image when this is built in the future.