_graph_key is null for default graph

[Oceania2018]

When allocate a new variable like W = tf.Variable(1.0, name="weight"), the default _graph_key should be filled with grap-key-0/. But in the TensorFlowNET.Examples LinearRegression, it's empty.

Python:

C#:

[dotChris90]

for traceability add the file location src\TensorFlowNET.Core\Variables\RefVariable.cs