Tensorflow to Sig downstreaming

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As we are already improving the upstreaming/promotion process with RFC template at #241 it could be useful to have something similar for downstreaming so that we could cover cases like tensorflow/addons#1781

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I think that we could also add an extra bullet point in https://github.com/tensorflow/community/blob/master/governance/api-reviews.md#experimental-apis before the deprecation option.

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tensorflow/addons#1944

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@seanpmorgan I suppose that we can close this as we are not handling this process anymore. We just accept downstreaming Issue/PR from TF teams