NXP backend: Use new Neutron flow for failing constant pad tests.

[MartinPavella]

Summary

In Neutron SW SDK 3.1.0, some edge cases of the constant pad operator are no longer supported by the old flow. Affected tests now use the new flow where the pad is supported.

(Only the last commit is relevant)

Test plan

Unit tests provided.

cc @robert-kalmar @JakeStevens @digantdesai

[pytorch-bot]

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❌ 1 Cancelled Job, 2 Unrelated Failures

As of commit af106a1 with merge base c8c791a ():

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest-buck / macos / macos-job (gh)
  ##[error]The operation was canceled.

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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[robert-kalmar]

Parametrize the test to it for legacy and new flow. Those two are distinct and the new mlir flow is experimental, hence the EIEX-855 is still valid.

Yes, eventually the EIEX-855 will be closed as outdated, once the mlir flow is mature enough but we are not yet there.
As we will do this for multiple operators define a mlir_flow as default parameter, like for qat - executorch/backends/nxp/tests/use_qat.py