Update addition example's python code for tf2.5

[dskkato]

I tried to migrate Python code using TensorFlow 2.x version.

In TensorFlow 2.x, the eager mode is ON by default, which makes it hard to get a concrete computation graph. One way to obtain a similar result is to wrap the function and use get_concrete_function method like in this PR.

Please refer to TF documents for detail : https://www.tensorflow.org/guide/function

I also tried to change outputs node name (which is Identity now), for compatibility, but could not find any solution regarding to this point. Please note that this PR makes the addition example totally incompatible with TensorFlow 1.x versions.

I'm glad to take any feedback. And if you already working on this point in a different way, please let it go.

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[dskkato]

@googlebot I signed it!

[adamcrume]

Thanks for the update! This looks good to me. I think it's fine if the examples require TensorFlow 2, since that's what the library is now targeting.

[ptitfred]

Please note that when ran as advertised (in the examples/addition directory) it creates another subtree examples/addition/examples/addition.