Make `nn.Transformer` TorchScript compatible (#28561)
Summary:
This makes `nn.Transformer` usable from TorchScript. It preserves backwards compatibility via `__setstate__` on the encoder/decoder.
Fixes https://github.com/pytorch/pytorch/issues/24173
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28561
Differential Revision: D18124753
Pulled By: driazati
fbshipit-source-id: 7314843e5aa9c9bf974c4672e4edb24ed8ef4a6f
