[Java] Graph environment decoupling in preparation of eager execution

[karllessard]

This PR is the first of a series of pull requests that enables eager execution in the TensorFlow Java client. It might look ambitious at first but is pretty basic when understanding what is coming next.

This PR decouples the actual implementation for building operations, which is tightly bound to graph execution, from its public interface. Doing this in a single PR helps to get a better picture of the targeted architecture and discuss about it if needed.

So in brief:

• Operation concrete class became a GraphOperation that implements the Operation interface by extending from the AbstractOperation abstract class
• OperationBuilder concrete class became a GraphOperationBuilder that implements the OperationBuilder interface
• Graph implements the ExecutionEnvironment interface to be used by the Ops classes

Once we are happy with this solution, we can start adding Operation, OperationBuilder and ExecutionEnvironment implementations for eager execution, which are pretty straightforward according to my previous POC.

I tried to preserve backward compatibility as much as possible. I ended up with the following exceptions:

• Constructor in Output is now package-private and accepts only AbstractOperation instances
  ** Note: I could have play around to keep it public but for me, it sounds wrong to instantiate an Output directly instead of calling the available type-safe Operation.output method

• Scope.graph() has been changed to Scope.env() to return an instance of ExecutionEnvironment instead instead of a Graph
  ** Note: This can only impacts clients already using the Ops API and even there, unless they did something tricky, the Ops classes will take care of the change flawlessly.

See diagram below for an overview of the targeted architecture:

[karllessard]

FYI, just applied a small change to this PR

[tensorflowbutler]

Nagging Reviewer @asimshankar: You have been added as a reviewer to this pull request. Please add your review or reassign. It has been 44 days with no activity and the awaiting review label has been applied.

[karllessard]

I think this should be assigned to @sjamesr instead.

[karllessard]

Thanks for approving the PR, @sjamesr

Since it was created a while ago, I had to rebase the code on current branch to get it compile, you might need to reapprove.

Also, I took that opportunity to rename GraphNode* classes to GraphOperation* for consistency with the upcoming eager implementation (i.e. EagerOperation*), it was bugging me a bit. I updated the diagram above accordingly, sorry for the trouble.

[karllessard]

Friendly reminder @sjamesr to please re-approve this PR,

Since it is a XL pull request, I'm just afraid that I'll need to rebase it more than once if we wait too long between approvals. Thanks!