Arm backend: Match fp32->int32 cast between pytorch and TOSA's CAST

[YufengShi-dudu]

• Pytorch .to() truncates values toward zeros, while TOSA's CAST rounds values to the nearest int. They will show different behaviours on fp32->int32 casting.
• Fix the mismatched behaviour with the following transformation:
  Before:
  output = to_copy(x, torch.int32)
  After:
  is_non_negative = x >= 0
  floor_x = floor(x)
  ceil_x = ceil(x)
  decorated_x = where(is_non_negative, floor_x, ceil_x)
  output = to_copy(decorated_x, torch.int32)

Change-Id: I21286432eeb0a5e2f21865f3ac097051c921a9b3

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12243

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❌ 1 New Failure, 1 Cancelled Job, 4 Unrelated Failures

As of commit 4e19b63 with merge base b342f83 ():

NEW FAILURE - The following job has failed:

• trunk / test-llama-runner-mac (fp32, coreml) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

• trunk / test-qnn-model (fp32, mb) / linux-job (gh)
  ##[error]The operation was canceled.

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• trunk / test-models-linux-aarch64 (mobilebert, xnnpack-quantization-delegation, linux.arm64.2xlarge) / linux-job (gh) (similar failure)
  ##[error]The operation was canceled.

BROKEN TRUNK - The following jobs failed but was present on the merge base:

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

• pull / test-models-linux (mobilebert, portable, linux.2xlarge) / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-models-linux (mobilebert, xnnpack-quantization-delegation, linux.2xlarge) / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• trunk / test-models-linux-aarch64 (mobilebert, portable, linux.arm64.2xlarge) / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[oscarandersson8218]

Unrelated CI failures. LGTM!