Migration of Python examples to TF2.5

[dskkato]

In a version 0.17.0 release, the back end of this crate became TF 2.5, while following example Python codes stay for TF 1.x series.

• addition - resolved by Update addition example's python code for tf2.5 #307
• regression
• regression_checkpoint
• regression_savedmodel - resolved by Update regression_savedmodel example for tf2.5 #312

In addition to those examples, I also would like to add some Keras examples like #286 since its signature usage was helpful for me.

Would you mind if I try these migrations? I already confirmed basic TF2.5 usages posted in my repository.

[adamcrume]

By all means, updating the Python to TF 2.5 is helpful and examples are always welcome!