Added echo state network (ESN) recurrent cell

[pedrolarben]

Part of #1809

I have been working with recurrent neural networks. I needed an implementation of Echo State Networks (ESN) for tensorflow. I found some implementations, but none of them was compatible with tensorflow2, so I implemented it. I think it would be great to add ESN to tensorflow_addons so everybody can use it.

This pull request add the recurrent ESNCell.

"Echo state networks (ESN) provide an architecture and supervised learning principle for recurrent neural networks (RNNs). The main idea is (i) to drive a random, large, fixed recurrent neural network with the input signal, thereby inducing in each neuron within this "reservoir" network a nonlinear response signal, and (ii) combine a desired output signal by a trainable linear combination of all of these response signals".
Herbert Jaeger, Jacobs University Bremen, Bremen, Germany

References

• The “echo state” approach to analysing and training recurrent neural networks-with an erratum note. Herbert Jaeger, 2001. Cited by 2281. [pdf]
• ESN implementation for tensorflow 1.12
• ESN implementations for pytorch: EchoTorch, pytorch-esn

[qlzh727]

Thanks for the change.

[qlzh727]

Thanks for the change. Please address the remaining comments.

[pedrolarben]

Thanks for your comments, I have done the changes.
Please, let me know if there is something missing.

[pedrolarben]

@qlzh727 Thank you very much for your help.
Once it is merged I will proceed with the remaining code from #1809 (the layer class).

[seanpmorgan]

Thanks!