Enable XNNPACK pybindings (#2152)

Summary:
Enabling pybindings for XNNPACK, this allows us to run XNNPACK Delegate's python tests in open source.

Pull Request resolved: https://github.com/pytorch/executorch/pull/2152

Test Plan:
```
CMAKE_BUILD_PARALLEL_LEVEL=9 EXECUTORCH_BUILD_XNNPACK=ON EXECUTORCH_BUILD_PYBIND=ON pip install . --no-build-isolation -v
```
After setup
```
python -m unittest backends/xnnpack/test/models/mobilenet_v2.py
```

Reviewed By: digantdesai, larryliu0820

Differential Revision: D54289433

Pulled By: mcr229

fbshipit-source-id: 2455a56eb349694fe9c4a78371acdf1f18d7a0d8