Last inference node does not have eval

[mbartling]

The very last node in every graph is a Placeholder so no is_eval gets set for output nodes. Should be an easy fix

[dboyliao]

Why does placeholder need evaluation?
According to the snippet, it's simply add a tensor into context object.
There is no operation to evaluate.

[dboyliao]

As for the output nodes, they need no evaluation in the code we generate.
The rationale here is that we leave the output tensor (nodes) untouched so the user can get the tensor later and manually evaluate whenever they like.