Build pybindings and link in backends when building pip wheels

[dbort]

Stack from ghstack (oldest at bottom):

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• 3476

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• -> #3470

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Always build the pybindings when building the pip wheel.

Always link in XNNPACK.

Differential Revision: D56857488

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/3470

• :page_facing_up: Preview Python docs built from this PR

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:x: 2 New Failures

As of commit c7416f7350f09be07c176bbc5900543bcaf4b3d2 with merge base 5d2a17b29f57ab77f0481ca1d96d827e610dc4bf ():

NEW FAILURES - The following jobs have failed:

* [Build Linux Wheels / pytorch/executorch / upload / manywheel-py3_8-cpu](https://hud.pytorch.org/pr/pytorch/executorch/3470#24487086548) ([gh](https://github.com/pytorch/executorch/actions/runs/8916099815/job/24487086548)) `Unable to find any artifacts for the associated workflow` * [Build M1 Wheels / pytorch/executorch / upload / wheel-py3_8-cpu](https://hud.pytorch.org/pr/pytorch/executorch/3470#24487078523) ([gh](https://github.com/pytorch/executorch/actions/runs/8916099733/job/24487078523)) `Unable to find any artifacts for the associated workflow`

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[dbort]

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@dbort merged this pull request in pytorch/executorch@7d5ba40a351031a541da1d3721ed2cb85d31e94d.