Arm backend: Remove # type: ignore from tosa_serializer import

[Sebastian-Larsson]

A leftover from TOSA 0.80 was the need for # type: ignore after the tosa serializer import. This ignore is now removed, along with moving the imports to the top of the file instead of having it as a dynamic import.

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

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• AWS was down, GHA infrastructure effected / recovering

❌ 7 New Failures, 3 Unrelated Failures

As of commit 5e60e2f with merge base 02b535f ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 7a0cea0ae23d58963df14c22da8a1477c10a2357aada7f5f02706e02a5052112 /exec failed with exit code 1
• pull / unittest / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 976e145b057098e579a2962183a95910bde20d33e874952dc2da1cfc79dfa62b /exec failed with exit code 1
• Test CUDA Builds / export-voxtral-cuda-non-quantized / linux-job (gh)
  RuntimeError: Command docker exec -t 4beac390aa9e922a4a5722f98565ad7d5a1e9964ad5b587b82d884e184f13714 /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-quantized-int4-tile-packed / linux-job (gh)
  RuntimeError: Command docker exec -t dfa1f06a3a4d51ebd9feee52e1029cb48a7d8b175c38d0bd574538cdabe880da /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-quantized-int4-weight-only / linux-job (gh)
  RuntimeError: Command docker exec -t 5f63167550cf5457386d995b54bba538458a411e2f46991ca58d6b064ac2aee0 /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

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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[oscarandersson8218]

CI failures seem unrelated