Cyphers/exec can create tensors (#3445) (#3462)
Cyphers/exec can create tensors (#3445) (#3462)

* Add method to check if an Executable can create tensors

* Add a Backend check to see if Executable can create tensors

* Update per review comment

* style

* Possible fix so that backend does not keep holding on to the dummy ng_exec
NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph