* adding unit tests

* ignoring failing tests

* removing ignore from unit tests

Unit testing the operation too.

Exposing also the `IsFinite` one.
Unit testing both operations.

Meant to be a shortcut to directly set some basic configuration.

E.g.

```
var sessionOptions = new SessionOptions();
sessionOptions.SetConfig(new ConfigProto
{
   LogDevicePlacement = true,
   InterOpParallelismThreads = 4,
   GpuOptions = new GPUOptions
   {
      AllowGrowth = true,
      PerProcessGpuMemoryFraction = 0.12
   }
});
var session = tf.Session(sessionOptions);
```

Adding `AbsGrad`

To be aligned with Tensorflow, introducing also `Sign` operation
and its gradient.

* removing unused namespaces

* implementing Tensor to scalar and scalar to tensor conversions

Unit testing the gradient too.

Also unit testing all the operator cases.

Also unit testing all the operator cases.

Change nest.flatten to generic.

Added continue statement to as that is what is in the original python…