Arm backend: Add TOSA dialect conversion ops

[oscarandersson8218]

Add fake implementations for CAST and move RESCALE and CAST_TO_BLOCK_SCALED into conversion_ops.py.

cc @digantdesai @freddan80 @per @zingo @mansnils @Sebastian-Larsson @robell @rascani

[pytorch-bot]

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❌ 1 Cancelled Job, 1 Unrelated Failure, 1 Unclassified Failure

As of commit c9a8c36 with merge base 6018283 ():

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• trunk / test-huggingface-transformers-macos (qwen3-0.6b|coreml_fp32_gpu|--quantize) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1

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• pull / unittest-nxp-neutron / linux-job (gh)
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FLAKY - The following job failed but was likely due to flakiness present on trunk:

• trunk / test-models-linux-aarch64 (softmax, portable, linux.arm64.2xlarge) / linux-job (gh) (matched linux rule in flaky-rules.json)
  docker: Error response from daemon: Head "https://registry-1.docker.io/v2/arm64v8/alpine/manifests/latest": Get "https://auth.docker.io/token?account=pytorchbot&scope=repository%3Aarm64v8%2Falpine%3Apull&service=registry.docker.io": context deadline exceeded (Client.Timeout exceeded while awaiting headers).

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