Created by: Xreki

In recurrent_grad op, inside grads need to be accumulated to the outside grad. In the current implementation, sum_ops are created to do this. If there are N inside grads, N - 1 sum_ops will be created, which each sum_op adds one inside grad to the outside grad.

In this PR, a single sum functor is called directly to add N inside grads.

This PR is tested in PaddingRNN padding model.