Fix HiFi conv2d_nhwc operator for depthwise case (again)

[mcremon-meta]

Summary: Seems like we let one case slip. Add a fallback for it.

Differential Revision: D98634082

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18559

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❌ 1 New Failure, 2 Unrelated Failures

As of commit b08eb3d with merge base 502d2de ():

NEW FAILURE - The following job has failed:

• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

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

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

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[meta-codesync]

@mcremon-meta has exported this pull request. If you are a Meta employee, you can view the originating Diff in D98634082.

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