NXP backend: Documentation for QAT

[pytorchbot]

Summary

Adds documentation NeutronQuantizer support for QAT.
Adds train support for QAT mode in calibrate_and_quantize function.

Test plan

Mostly documentation related changes - no tests added.
The quantization related function is partially covered by already existing tests.

cc @robert-kalmar @JakeStevens @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18282

• 📄 Preview Python docs built from this PR

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❌ 1 New Failure, 2 Cancelled Jobs

As of commit 030fac2 with merge base 8c0a60b ():

NEW FAILURE - The following job has failed:

• Test CUDA Windows Export and E2E / export-model-cuda-windows-artifact (nvidia, diar_streaming_sortformer_4spk-v2, non-quantized) / linux-job (gh)
  RuntimeError: Command docker exec -t a3e148882bf49c1e5169dd347642542bb9b232857c12dfa57ef6374229e18893 /exec failed with exit code 1

CANCELLED JOBS - The following jobs were cancelled. Please retry:

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

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