minor refactor of dag_engine.h

[mli]

there are some suggestions potentially to make DAGEngine more readable:

(Variable, OprHandle) -> (VarPtr, OprPtr) or (VarHandle, OprHandle)
(use_var, mutate_var) -> (const_var, mutable_var) or (read_var, write_var)
(NewVar, NewOperator) -> (NewVar, NewOpr) or (NewVariable, NewOperator)

i'm not too sure about the following two

(DeleteOperator, PushDelete) -> (DeleteOpr, DeleteVar)
(Var, Opr, Fn) -> (Var, Oper, Func)

do we need both Push(Fn exec_fun) and PushAsync? it looks like we can always first NewOperator and then Push(OprHandle)

[hotpxl]

I think the suggestions are good. One thing about mutate_var is, even if you need to READ and WRITE a variable, you should not put it into use_var once it's in mutate_var. The name const_var and mutable_var would be better in this sense.

PushAsync pushes a temporary operator. It's needed because I use it to delete an operator.

[mli]

got it, so the difference is that

the user need to delete the op in Create & Push(op), while the system managers the op in Push/PushAsync(func)? i'm fine with that. another solution maybe always return std::shared_ptr<OprHandle>

[hotpxl]

Yes.

We actually have to do some work (push OprHandle into queue) before destructing it. So a destructor would not meet the needs.

[hotpxl]

Also renaming DAGEngine to Engine, in case I forget.