Arm backend: Reduce complexity of get_model_and_inputs_from_name

[Sebastian-Larsson]

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

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❌ 2 New Failures, 1 Cancelled Job

As of commit ab3ef94 with merge base 2c706f1 ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 62be21dddbad0e3aab758e27d656da23fce53ab98b20abd73fb42152a003eab4 /exec failed with exit code 1
• Test CUDA Builds / export-voxtral-cuda-artifact / linux-job (gh)
  RuntimeError: Command docker exec -t d0ca954d2bc9ef5aa07a5eaa027602b3ae68441dabc5474dd56b6aecfa5ae427 /exec failed with exit code 2

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

• trunk / test-llama-runner-mac (fp32, xnnpack+custom+quantize_kv) / macos-job (gh)
  ##[error]The operation was canceled.

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