Build pybindings and link in backends when building pip wheels (#3470)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3470

Always build the pybindings when building the pip wheel.

Always link in XNNPACK.

imported-using-ghimport

Test Plan: Imported from OSS

Reviewed By: mikekgfb

Differential Revision: D56857488

Pulled By: dbort

fbshipit-source-id: 96913201850b3c0d64f7874a87b42066b352d492