Update the example of exporting Bart + BeamSearch to ONNX module to resolve comments.
This is a PR to resolve all of comments left in previous PR:

https://github.com/huggingface/transformers/pull/13765

Also provides a README.md for this example.
@NielsRogge  @garymm  

Please help to take a review. Thanks.<|||||>@michaelbenayoun 
 Could you please help to take a review? Thanks!<|||||>@garymm Please help to take a review.<|||||>Thanks for integrating all the feedback @fatcat-z ! Is it OK if I merge this?