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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❌ 3 New Failures

As of commit 9fdda5d with merge base 7b5c60d ():

NEW FAILURES - The following jobs have failed:

• Build Cadence / cpu-x86 / build (gh)
• Lint / link-check / lint-urls (gh)
  Process completed with exit code 1.
• pull / unittest-editable / macos / macos-job (gh)
  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.