Add pybind API reference docs (#6276)

Summary:
[pybind] Add pybind API reference docs

 As titled

Pull Request resolved: https://github.com/pytorch/executorch/pull/6276

Test Plan: https://docs-preview.pytorch.org/pytorch/executorch/6276/runtime-python-api-reference.html

Reviewed By: dbort

Differential Revision: D64449105

Pulled By: larryliu0820

fbshipit-source-id: 6a0d09510ff2a685c7d98a0119158a5d47d619be