Added fix for issues with saved model in strip_custering()

[SaoirseARM]

This PR fixes a bug in strip_clustering() where cluster nodes remained in the model when exporting to saved_model format.

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[SaoirseARM]

@googlebot I signed it!

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CLAs look good, thanks!

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CLAs look good, thanks!

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[akarmi]

@alanchiao, can you please check this change and merge it if it looks OK to you.

[alanchiao]

@SaoirseARM: actually taking a closer look, this isn't unit tested. Can you add unit testing?

@Ruomei also mentioned that testing with this PR enables conversion of a clustered model to TFLite via from_keras_model in TF2, which should also be unit tested so that it continues to work.

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[SaoirseARM]

Hi @alanchiao , I have added two tests. One integration test including conversion to tflite and one unit test to ensure the stripped kernel is correct.

[alanchiao]

Feel free to cleanup in a followup PR: os.remove should not be necessary since tempfile will automatically delete it.

[alanchiao]

Thanks @SaoirseARM. Left a comment for the future.

Merging this PR for now since other work is depending on it.

[alanchiao]

@SaoirseARM: actually last thing. Could you squash all your commits into one so that there is a clean commit history? Doing the cleanup should generally be done prior to merging a PR.

[SaoirseARM]

Thanks @alanchiao , I have squashed commits now down to one. Please let me know if I should update anything else.