Fix weight cache bug

[GregoryComer]

Summary: XNNPACK sometimes calls weight cache look_up_or_insert with ptr==nullptr. This is a bit weird - maybe a bug on the XNNPACK side?, but it seems to do this when it's already hit the cache. Just return the cached value for the key without validating contents in this case, since we can't insert anything.

Differential Revision: D97006279

[pytorch-bot]

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❌ 2 New Failures, 5 Pending, 2 Unrelated Failures

As of commit bb64d12 with merge base eb92cec ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model
• pull / unittest-editable / linux / linux-job (gh)
  examples/models/test/test_export.py::ExportTest::test_mv3_export_to_executorch

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]

@GregoryComer has exported this pull request. If you are a Meta employee, you can view the originating Diff in D97006279.

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[meta-codesync]

@GregoryComer has imported this pull request. If you are a Meta employee, you can view this in D97006279.

[digantdesai]

worst case we will return SIZE_MAX?
And we can't continue without ptr from here anyway.

[GregoryComer]

Test failures are a coreml-related and portable tolerance check. Bypassing.