Expose is_stateful in ops

[rnett]

Op protos have a is_stateful, which should be exposed at the instance level in the generated operand classes. This is needed in function defs as StatefulPartitionedCall should be used instead of PartitionedCall to call it.

[Craigacp]

I guess that's the thing we should use to determine if an op needs a strong or a weak reference in eager session.

[rnett]

Maybe? I'm not entirely clear on what it is used for, but that sounds right.

I've worked around this using the op def list in TensorFlow, and added a more efficient function for it in #233.