Breaking down the Layers.swift file

[Shashi456]

So the layer.swift file is long and since we are actively working on adding layers #54 , it might make more sense to break down the file into convolutions, pooling, recurrence and other such files. Might make the code overall more readable.
@rxwei @tanmayb123 Thoughts?

[rxwei]

Sounds reasonable to me. Feel free to send a PR.

[eaplatanios]

Also started breaking down the operators file in #70 , so it may make sense to follow a similar pattern and create a Layers directory.