Qualcomm AI Engine Direct - Add test verifying LLM calib/eval limits

[DannyYuyang-quic]

Summary

Add TestExampleLLMScript.test_static_llm_eval_limit, which compiles a static
LLM once with a fixed calibration, then evaluates the same pre-generated pte
twice with different eval limits and asserts the resulting wikitext perplexity
differs.

Test plan

python backends/qualcomm/tests/test_qnn_delegate.py   TestExampleLLMScript.test_static_llm_eval_limit  -s  ${SERIAL_NUM} -m SM8750   -b build-android   -a .   --executorch_root .   --model_name smollm2_135m

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20234

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❌ 2 New Failures, 1 Cancelled Job, 1 Unclassified Failure

As of commit a3557d4 with merge base 7282106 ():

NEW FAILURES - The following jobs have failed:

• Cadence Build & Test / hifi-build / hifi4 (gh)
  Input required and not supplied: aws-region
• Cadence Build & Test / vision-build / vision (gh)
  Input required and not supplied: aws-region

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• periodic / gather-models (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)

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

• pull / unittest / macos / macos-job (gh)
  ##[error]The operation was canceled.

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[DannyYuyang-quic]

@pytorchbot label "release notes: qualcomm"

[DannyYuyang-quic]

@psiddh Hi, this is a follow-up PR to #20113

Could you please take a look? Thanks!