Simplify api for building Layers

[juliabeliaeva]

This pull request attempts to simplify Layer building api by doing the following:

1. Unify functional and sequential functions signatures.
2. Remove separate output shape computation (btw, it was computed incorrectly in ZeroPadding1D and Concatenate layers).
3. Combine Layer#build and Layer#forward functions.
4. Combine GraphTrainableModel#build and GraphTrainableModel#forward functions.

This change was proposed as a part of Layer api simplification. Apart from the benefits described in linked proposal, having a single build method will make it easier to split Layer into an immutable Layer containing variables and layer output and a reusable LayerBuilder for building the Layer.
Possible downsides so far consists of one thing: the api no longer will mimic Keras api and it would be more difficult for contributors to borrow its code to implement new layers.

[ermolenkodev]

Looks great to me! Excellent that it's not needed to calculate output shape manually anymore.