Implement a portable adaptive_avgpool2d kernel

[GregoryComer]

Summary

Add a basic _adaptive_avgpool2d kernel to portable. It shows up in some CV models.

cc @larryliu0820 @manuelcandales

[pytorch-bot]

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❌ 1 New Failure, 5 Pending, 4 Unrelated Failures

As of commit 2576b9b with merge base be0cdee ():

NEW FAILURE - The following job has failed:

• Build Presets / windows (windows) / build (gh)

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-editable / windows / windows-job (gh) (similar failure)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 1

BROKEN TRUNK - The following jobs failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Build Presets / windows (pybind) / build (gh) (trunk failure)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 1
• pull / unittest / windows / windows-job (gh) (trunk failure)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_linear_model

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[meta-codesync]

@GregoryComer has imported this pull request. If you are a Meta employee, you can view this in D95997748.

[Gasoonjia]

why do we need a underscore at the front? is this kernel internal only and won;t expose to user?

[GregoryComer]

Good question. This is actually the ATen op name, so this is referencing the official schema here: https://github.com/pytorch/pytorch/blob/main/aten/src/ATen/native/native_functions.yaml#L12443. I'm not sure why the ATen op itself starts with an underscore.