NXP backend: Enable aten.softmax.default delegation to Neutron.

[MartinPavella]

Summary

This PR enables the delegation of aten._softmax.default to Neutron.

Test plan

Unit-tests provided.

cc @robert-kalmar @JakeStevens @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17368

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❌ 3 New Failures

As of commit ed9f316 with merge base 08282a4 ():

NEW FAILURES - The following jobs have failed:

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh)
  RuntimeError: Command docker exec -t 09e686c6191783fd5bda16cfccf203411922b4f5f18000dd00c101c489f28bca /exec failed with exit code 137
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t c8481e6a8440c908e995454385b6f90ff3fb82a8deb98c40edf6ed20e8472294 /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t d8a35e513229f57db2d0e6b7fddf9adac1de4eba4fd18a5ad21a2d15c2542665 /exec failed with exit code 1

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