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

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

Author: Gasoonjia

backends/qualcomm/_passes/lpai_partition_fallback_support.py

@@ -314,6 +314,8 @@ def handle_back_to_back_nodes(self, graph_module: torch.fx.GraphModule):
         graph_module.graph.eliminate_dead_code()
 
     def call(self, graph_module: torch.fx.GraphModule) -> PassResult:
+        if self.compiler_specs is None:
+            return PassResult(graph_module, False)
         self.preserve_io_qdq(graph_module)
         unsupported_nodes = self.get_unsupported_nodes(graph_module)
         for node in unsupported_nodes: