Introduce pybind11 and build extension/pybindings/portable_lib (#332)

Summary:
Build Pybind11 shared library and load it into python environment. This way we can use `Module` python API to run end2end tests.

Add `test_end2end.py` in `pytest.ini` so that we have e2e test coverage.

Pull Request resolved: https://github.com/pytorch/executorch/pull/332

Reviewed By: tarun292

Differential Revision: D49254089

Pulled By: larryliu0820

fbshipit-source-id: cc3698bdd6620176d64e14027a37ed5b9897704a