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

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.

首先查看 Java 图和 eager 操作 API，然后阅读 issue #302 和 tensorflow/tensorflow#50816 中的依赖项。完成的标准是定义并实现一种在图之间或复制到 eager 操作构建器中的操作复制方式，包括可能的输入替换，并通过测试覆盖受支持的情况。