work around lack of optional output for forks (#429)

Summary:
Pull Request resolved: https://github.com/pytorch/translate/pull/429

Pull Request resolved: https://github.com/pytorch/fairseq/pull/618

PyTorch export for transformer models was broken because as written, they used a placeholder `None` value during inference for the variable `key_padding_mask` to indicate no padding, but PyTorch is unable trace such values. This diff adds a minor hack to allow the use of an empty tensor for the same purpose.

Reviewed By: jmp84

Differential Revision: D14581730

fbshipit-source-id: 2ea4664c20ecab8478c578b2182a85319140036c