Fix some recursive functions (e.g., reorder_incremental_state) to only touch...

Fix some recursive functions (e.g., reorder_incremental_state) to only touch each module once (#379)

Summary:
This can happen if a module is registered in more than one place in the network.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/379

Differential Revision: D13154498

Pulled By: myleott

fbshipit-source-id: a35575d1956a46cd35ac8b16a719ad20ac3e380a