onnxruntime

Update optimizers to be able to utilize a constant initializer from an ancestor graph (#1346)

* Now that we check for a constant initializer in an ancestor graph we also need to be able to retrieve and replace that initializer.
Add helpers to do so.
Update optimizers to use the new helpers.
Fix bug in UnsqueezeElimination where it wasn't checking if the initializer it was replacing was constant.