For input argument with a list of variables, drop_empty_grad is not allowed because it makes the correspondence bewteen a variable and its gradient ambiguous. Use REGISTER_OP_EX to register the op or call InputGrad(?,false) in GradOpDescMaker.
* Conditional Block Forward
* Assign Operator.
Out=X, when type in [LoDTensor/SelectedRows/LoDTensorArray]
* Stash
* Add Scope::Rename
it is useful in gradient phase of an operator with block
* ConditionalBlock Grad Done
* Add comments
* yapf format code