Provide multiple versions of TensorFlow documentation

It would be very helpful if we get to select the version of Tensorflow for which the results are displayed. There have been some major changes from Tensorflow 1.X to 2.0. There are many developers who have mastered 1.X version and still use that version in their daily work. Also, the Tensorflow site doesn't provide such elegant documentation of previous versions the way you have displayed here. So it would be really helpful if this come to the devdocs.

该 issue 没有 указ明任何文件、测试或入口点。首先调查 DevDocs 如何获取和选择 API 文档版本；完成的标准是用户可以选择 TensorFlow 1.x 或 2.0，并查看相应的文档。

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