Fix meta kernel for conv2d nchw to ensure that groups divides channels

[DrJessop]

Summary: As titled

Differential Revision: D87305499

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15866

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✅ You can merge normally! (2 Unrelated Failures)

As of commit d237512 with merge base 179a155 ():

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 / linux / linux-job (gh) (trunk failure)
  exir/backend/test/test_lowered_backend_module.py::TestBackendAPI::test_emit_nested_lowered_backend_module
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
  exir/backend/test/test_lowered_backend_module.py::TestBackendAPI::test_emit_nested_lowered_backend_module

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[meta-codesync]

@DrJessop has exported this pull request. If you are a Meta employee, you can view the originating Diff in D87305499.

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