Add ETDumpGen APIs to pybindings (#1272)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1272

We want to generate an ETDump file when running the Executorch program with
a different delegate, but we use the python API for e2e tests which doesn't have the
option of creating a dump.

Add this option when creating the Executorch module, and add an API for writing the
dump out to a file.

Reviewed By: tarun292

Differential Revision: D50866783

fbshipit-source-id: af059f5acab79a7696237b6db7686d22138e3120