Modify BeamSearch to support CharSourceEncoder (#18107)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18107
Pull Request resolved: https://github.com/pytorch/translate/pull/396
also:
1. fix issues with OptionalType not having a createWithContainedType (PyTorch diff)
2. Delete tests for ONNX full beam search export (nobody is using it and it just makes things harder. Currently ONNX doesn't support `_unwrap_optional`)
Reviewed By: jmp84
Differential Revision: D14483771
fbshipit-source-id: 0e37ef1cb5a16d03a535eef808b0488b98802128
