ability to add custom input feautures

[ghassen-fatnassi]

we found that it's very limiting to have input feature of the network baked and not flexible , so we added a way to add features by writing their equation explicitly , good for experimenting how the network actually learn true and useful repesentations like tan(x).

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.