Arm backend: Refactor partitioning of int64

[Erik-Lundell]

Earlier logic partitioned some edge cases that
could not be handled by the backend, and caused small single-constant partitions.

The new logic is more conservative, so some
cases that could be partitioned before might not be anymore. However, the idea is to trade a little less partitioning for more robustness.

Update tests affected by the change.

Also add small dtype debug info to aid with future int64 bug squashing.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

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❗ 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• Multiple CI trunk failures after landing https://github.com/pytorch/pytorch/pull/161002

❌ 5 New Failures, 1 Unrelated Failure

As of commit 505604a with merge base b4e1145 ():

NEW FAILURES - The following jobs have failed:

• Apple / build-benchmark-app / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Apple / build-demo-ios / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Apple / build-frameworks-ios / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Build documentation / build (buck2) / Build doc (gh)
  At least one of the pre-conditions you specified did not hold
• trunk / test-coreml-delegate / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• trunk / test-huggingface-transformers-coreml (qwen3-1.7b|xnnpack|--quantize) / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 139

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[YufengShi-dudu]

Approved internally

[YufengShi-dudu]

Unrelated failure in unittest-arm-backend-with-no-fvp (test_pytest_ops) / linux-job and test-arm-backend (test_pytest_ops_ethosu_fvp) / linux-job:
backends/arm/test/ops/test_sigmoid_16bit.py::test_sigmoid_tosa_INT_add_sigmoid[ramp] - AttributeError: 'GraphModule' object has no attribute 'output_node'

[zingo]

Sigmoid error is fixed I triggered a rebase to make sure.

[zingo]

fails are unrelated to Arm backend