transformer onnx trace: skip no-op transpose (#333)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/333

A tiny hack to speed up inference slightly for transformer beam search after export to graph mode. Specifically, there is no need to transpose a dimension with size 1 (the sequence length of a single decoder time step during beam search) with its neighbor immediately before a view/reshape.

Reviewed By: jmp84

Differential Revision: D12833011

fbshipit-source-id: f9c344a9ad595e6e48a8a65b31cf2b1392f9b938