render 3rd party module annotations as forward references

render 3rd party module annotations as forward references

Fixed issue where registration of custom ops was prone to failure due to
the registration process running ``exec()`` on generated code that as of
the 1.7 series includes pep-484 annotations, which in the case of end user
code would result in name resolution errors when the exec occurs. The logic
in question has been altered so that the annotations are rendered as
forward references so that the ``exec()`` can proceed.

Change-Id: I142b89e2f6354b2ce253f3b8109f59fa3aae68ef
Fixes: #920