Encapsulate the state of stateful networks

[mvrahden]

Is your feature request related to a problem? Please describe.
The current implementation of stateful networks exposes implementation details to the user of those architectures, such as the previous state and the graph.

Describe the solution you'd like
To resolve this, encapsulate the graph and the state of the network during forward pass, e.g.:

net.forward([0, 1]); /* or with custom learning rate: net.forward([0, 1], 0.3) */

The encapsulation of the graph and the state will bring the API more close to what we see from the stateless architectures.

Describe alternatives you've considered
none

Additional context
none