Add pybind extension to the DLL search path on Windows

[GregoryComer]

Summary

When installing ExecuTorch from a wheel on Windows, the native pybinding extension DLL fails to load (#14443). This is because it's not on the search path. From a quick Google search, it seems like the recommended way to handle this is to call os.add_dll_directory prior to loading the extension.

I tested this locally by installing from wheel in a clean env and then patching the installed copy in site-packages. With this change, I was able to import executorch.extension.pybindings.portable_lib. I also ran the XNNPACK add op tests as a sanity check, which use pybindings.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14446

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❌ 2 New Failures

As of commit b3933d8 with merge base 90ee347 ():

NEW FAILURES - The following jobs have failed:

• pull / test-qnn-wheel-packages-linux (3.11) / linux-job (gh)
  RuntimeError: Command docker exec -t 0c4fbcb83a2cb5afe54edb569da2903cc3a72cc5459a3e5a852f75ad321ff5ab /exec failed with exit code 1
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 2088b36ed628c3e6e9b0109eca3e9acd2928b25e120849ef2fbb64fcb1f16b7c /exec failed with exit code 1

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[mergennachin]

if sys.platform.startswith("win"):

so that it can catch "win64" and "windows"

[GregoryComer]

Win32 is actually the stable value for sys.platform for both 32-bit and 64-bit Windows builds. I'm happy to change it, though it seems like direct comparison to "win32" is the idiomatic appoach.