Arm backend: Replace asserts with report_reject in operator_support

[Sebastian-Larsson]

Remove redundant check node.target in self.targets, as well as replacing asserts with proper report_reject. This way the graph won't be stopped from lowering, but the operators will instead end up on CPU.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

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❌ 3 New Failures, 1 Unrelated Failure

As of commit 9e0e4aa with merge base b02db12 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t d386641391c19c3e0af682be00e83e0bfe4efc6d93afb4d7cb2646e39a9eb323 /exec failed with exit code 1
• trunk / test-models-linux-aarch64 (qwen2_5, portable, linux.arm64.2xlarge) / linux-job (gh)
  RuntimeError: Command docker exec -t 208808c036c529d887535670a5c4e00487055d366651d749c6606642a9e2858a /exec failed with exit code 1
• trunk / test-models-macos-cpu (llama3_2_vision_encoder, portable) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/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-models-macos-cpu (ic4, xnnpack-quantization-delegation) / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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[zingo]

Fails unrelated