NXP backend: Enable constant_pad_nd with new Neutron flow.

[MartinPavella]

Summary

Enable constant_pad_nd with new Neutron flow in NXP backend. Reflect the new Neutron requirements and verify correct function.

Test plan

Unit tests provided.

cc @robert-kalmar @JakeStevens @digantdesai @rascani

[pytorch-bot]

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✅ You can merge normally! (2 Unrelated Failures)

As of commit 28bb54c with merge base 5971a4b ():

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)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_8a4w_recipe
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/recipes/test_xnnpack_recipes.py::TestXnnpackRecipes::test_8a4w_recipe

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[MartinPavella]

@novak-vaclav as per usual.