* implement math_ops.real, reduce_variance, reduce_std

* add dtype.real_dtype()

* add outward-facing api functions

2. 修正一个缺少参数处理的bug

Relative unit test in `OperationTest`.

Relative unit test in `OperationTest`.

Relative unit test in `OperationTest`.

Relative unit test in `OperationTest`.

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.