Add Identity initializer

[zaleslaw]

We are missing some initializers to fully support the export of models from Keras. One of them is the Identity initializer.

Add an initializer class, write documentation for it, write a test for it, try, if possible, create a small trainable network with it (in your own GitHub) and attach a link.

Also, support for export and import of initializer in JSON format should be added (see ModelLoader.kt and ModelSaver.kt)

A detailed description of the initializer can be found here

[hbrammer]

Hi there 👋🏻
I'm new to the party and would love to help.
Created a first draft of this in #73

I will hopefully complete this in the next couple of days.