Update tensorflow import in AI_platform_demo.ipynb

[safgen]

As the default runtime in Google colab is the latest (currently 2.4), direct import creates an incompatible "eeified" version. As a result the AI platform version create with the eeified model fails. So, I am suggesting this change so that others can run this tutorial without any hiccup.

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[safgen]

@googlebot I signed it!