add distributed training example of using TF 2.1 Strategy API

[jazzsir]

. add multi-worker strategy example to keras-API directory.
. move existing strategy example to estimator-API directory, to distinguish strategy examples.

Fixes #953

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[kubeflow-bot]

This change is 

[k8s-ci-robot]

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[jazzsir]

@googlebot I signed it!

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[coveralls]

Coverage remained the same at 96.512% when pulling adbc60f on jazzsir:master into 562d50f on kubeflow:master.

[gaocegege]

/ok-to-test

[gaocegege]

/lgtm
/approve

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