concat layer example

[harold]

From this google group post: https://groups.google.com/forum/#!topic/clojure-cortex/AtE-kAQCO8Y

We could have some examples of using concat (and the other layer graph operations) as tests with some .md descriptions in the docs referencing the tests.

The tests could go into experiment, or perhaps into the train tests (or layer tests? but maybe that's too low level).

@charlesg3 @cnuernber --- relevant to your interests.

[cnuernber]

Starting about here:
https://github.com/thinktopic/cortex/blob/master/src/cortex/verify/nn/layers.clj#L659

There are examples of using the graph-level layers (split, join, concatenate).

I think the crux of it is that each layer description object implicitly depends on the description before it unless the parents are manually specified.

So the description system does let you create some level of arbitrary graphs.