TFOutput>>#operation has graph in nil

[jvanecek]

Case reproduced in this commit

I updated TFOutputTest>>#testOperation to show the case: while TFOutput>>#operation and TFOutput>>#operationOn: are apparently the same TFOperation, the first one has the instance variable graph in nil and can't be used in any example defined in LibTensorFlowExamplesApp.

Would be nice them to be the same, or only having one way to ask for the operation and avoiding any confusion