Feature request: An option to return a sorted top-k.

https://github.com/clab/dynet/pull/1328 currently returns unsorted top-k values and the corresponding indices.
It'd be great if there is an option to return sorted top-k values and the indices.

Start by reading pull request 1328, which introduced the unsorted top-k values and corresponding indices, and locate the implementation and any tests it added. The work is complete when an option returns top-k values in sorted order while keeping each index paired with its value; update or add tests for that behavior.

Written by the indexing model from the issue text.