Arm backend: Fix arg-type MyPy errors

[Sebastian-Larsson]

Introduce function ensure_type() which throws an exception if the expected dtype is incorrect. This solves a number of mypy errors spread around the codebase.

Additionally, fix arg-type mypy errors.

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

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❌ 9 New Failures, 3 Unrelated Failures

As of commit f88d250 with merge base fdfeaa4 ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 9b331d0ffc9d15eb2dcd5d71b77f94566c0da7047f45b4533fa90392cc2e16d8 /exec failed with exit code 1
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 54ca991171bf50f90d0ece6b11e2083898c43e52a2df7551f942c6840910b4ee /exec failed with exit code 1
• Test CUDA Builds / export-gemma3-cuda-non-quantized / linux-job (gh)
  RuntimeError: Command docker exec -t 6b879e417a9431ebf0d8a8f3b0f260ce46c5bcdfe3d5267b40b6bf866db4dac2 /exec failed with exit code 2
• Test CUDA Builds / export-gemma3-cuda-quantized-int4-tile-packed / linux-job (gh)
  RuntimeError: Command docker exec -t 1476d917592f0e8f7c4e92e87ed8176b6810eb7a618c19f40268d635571d150e /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-non-quantized / linux-job (gh)
  RuntimeError: Command docker exec -t 0d73eb840d5dab2dfab577dd484aa3dc3160f95c2032b45766a10f9388553dea /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-quantized-int4-tile-packed / linux-job (gh)
  RuntimeError: Command docker exec -t 105ee63b505394bb95dcdf528e58d40c66ea35c8f56218de3f270061c4acd7ec /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-quantized-int4-weight-only / linux-job (gh)
  RuntimeError: Command docker exec -t e5fa7b1951b133df880dcb766e8b71f595946a4fb5058bd81bf914055d636fe5 /exec failed with exit code 2
• Test Metal Backend / export-voxtral-metal-artifact / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 2
• trunk / unittest-release / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_linear_model

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 / linux / linux-job (gh) (trunk failure)
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
• trunk / unittest-release / linux / linux-job (gh) (trunk failure)

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