Add method to copy graph operation to new graph or eager operation builder

[rnett]

There are a number of situations that I have ran into where I need to copy a graph operation to another graph operation, or an eager operation, sometimes substituting inputs. The most important is lifting things to init scopes in functions (it's graph -> eager). This depends on being able to access attributes and input lists (#302) and being able to set them (tensorflow/tensorflow#50816, not 100% necessary but makes things much easier). I plan on implementing it myself once those are both merged, this is a reminder and documentation if someone else ends up doing it.