fix make_positions() typo (#316)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/316

This code should actually be keeping the padded positions as `padding_idx` (though note that this is on the ONNX export path, and it has no effect in the most common case when using the exported network to do un-batched inference).

Reviewed By: myleott

Differential Revision: D10431872

fbshipit-source-id: 79fe4ac27cafcd4701e0f2a90e29d1b7362dc6f8