restructure bp export example (#751)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/751

Update the logic of bp export example to make it more understandable.

This diff tries to make the example more structural. Here `save_bundled_program` only takes charge for generating bp, without any I/O preprocessing. All other stuffs, including emit ET program. I/O preprocessing, etc, have moved under `export_to_bundled_program` function. We also make the code less tricky.

Reviewed By: dbort

Differential Revision: D50100494

fbshipit-source-id: 9c6e4c124e62325d30c005e65b508492cfb01994