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.

- Concat
- LGamma
- Tanh
- Slice

* removing unused namespaces

* implementing Tensor to scalar and scalar to tensor conversions

Unit testing the gradient too.

Add missing `operator -`s

Also unit testing all the operator cases.

Add missing `operator +`s

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…

Fixed behavior of UpdateSHA