Fix hybrid transformer state dict update after encoder layernorm rename (#633)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/633

Pull Request resolved: https://github.com/pytorch/translate/pull/456

This diff makes it easier to upgrade the state dict for components that use TransformerEncoderLayer

Reviewed By: jhcross

Differential Revision: D14916941

fbshipit-source-id: 6d0258c8a9492a720684dadce59c90fc87cbf5cf