Add `transform_exported_program` stub to ease renaming of `transform` method (#135)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/135

To avoid using co-dev mode, I am adding a helper that automatically tries both _transform and transform on ExportedProgram. Then I will be able to change OSS code without breaking internal tests.

Reviewed By: angelayi

Differential Revision: D48668701

fbshipit-source-id: 20758420a20c9d0134ed88882838a3f7b1039e51