Add a pure python wrapper to pybindings.portable_lib (#3137)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3137

When installed as a pip wheel, we must import `torch` before trying to import the pybindings shared library extension. This will load libtorch.so and related libs, ensuring that the pybindings lib can resolve those runtime dependencies.

So, add a pure python wrapper that lets us do this when users say `import executorch.extension.pybindings.portable_lib`

We only need this for OSS, so don't bother doing this for other pybindings targets.

Reviewed By: orionr, mikekgfb

Differential Revision: D56317150

fbshipit-source-id: 920382636732aa276c25a76163afb7d28b1846d0