Skip LpaiPartitionFallbackSupport when compiler_specs is None (forward fix for D108707519) (#20324)

[Gasoonjia]

Summary:

Forward fix for D108707519 (Qualcomm AI Engine Direct - LPAI Support Partition, #19835).

The new LpaiPartitionFallbackSupport pass crashes when run without QNN compiler
specs:

lpai_partition_fallback_support.py:153 get_unsupported_nodes
op_validator = QnnOperatorSupport(compiler_specs=self.compiler_specs, ...)
TypeError: 'NoneType' object is not iterable

The edge-transform pass framework injects compiler_specs into passes but
defaults it to None (QnnPassManager.get_to_edge_transform_passes(compiler_specs=
None)). Lowering paths that run the edge transform without partitioner-supplied
specs — e.g. the modai LPAI offline-compile path
(modai/test:test_qualcomm_lpai_recipes::test_export_and_lower_8a8w_writes_pte) —
reach this pass with compiler_specs=None, and QnnOperatorSupport then iterates
the None specs and aborts the whole to_edge_transform_and_lower.

The pass fundamentally needs compiler specs to decide which nodes are LPAI-
unsupported, so with no specs there is nothing to validate against. Skip the
pass (return PassResult(.., modified=False)) in that case, restoring the
behavior from before this pass existed for spec-less paths. On-device paths that
do supply compiler_specs are unaffected.

Differential Revision: D108853837

[pytorch-bot]

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✅ You can merge normally! (3 Unrelated Failures)

As of commit 4c9fa14 with merge base 48ff29e ():

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

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

• pull / test-arm-backend-no-driver (test_pytest_models_tosa) / linux-job (gh) (trunk failure)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)
• pull / unittest-nxp-neutron / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[meta-codesync]

@Gasoonjia has exported this pull request. If you are a Meta employee, you can view the originating Diff in D108853837.

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[shewu-quic]

++ @winskuo-quic for awareness