Fix embedding lowering crash when QNN_SDK_ROOT is unset (#21051)

Summary:
D110960687 (#20686) rewrote `Embedding.define_node` to
select between the optimized and legacy pcq-embedding lowerings based on
`is_qnn_sdk_version_less_than("2.48")`. That helper resolves the SDK
version via
`get_sdk_build_id`, which builds a path from
`os.environ["QNN_SDK_ROOT"]`.

`define_node` runs during AOT partitioning (`is_node_supported`), and
AOT-only
environments do not necessarily have `QNN_SDK_ROOT` set. In that case
`os.path.join(os.environ.get("QNN_SDK_ROOT", None), ...)` raised
`TypeError: expected str, bytes or os.PathLike object, not NoneType`,
breaking
every QNN lowering that contains an embedding — including the internal
`test_dummy_llama_qnn_16a4w_aot_and_runtime`.

Fall back to the legacy embedding lowering (valid on all QNN versions)
when
`QNN_SDK_ROOT` is unavailable, so the version-gated optimization is only
taken
when the SDK version can actually be determined. Also make
`get_sdk_build_id`
raise a clear `EnvironmentError` instead of a cryptic `TypeError` when
the
environment variable is missing.

This diff was authored with Claude Code.

Differential Revision: D112944232

Author: psiddh

backends/qualcomm/builders/op_embedding.py

@@ -3,6 +3,7 @@
 #
 # This source code is licensed under the BSD-style license found in the
 # LICENSE file in the root directory of this source tree.
+import os
 from typing import Dict
 
 import executorch.backends.qualcomm.python.PyQnnManagerAdaptor as PyQnnManager
@@ -242,6 +243,10 @@ def define_node(
         node: torch.fx.Node,
         nodes_to_wrappers: Dict[torch.fx.Node, PyQnnManager.TensorWrapper],
     ) -> PyQnnManager.PyQnnOpWrapper:
-        if is_qnn_sdk_version_less_than("2.48"):
+        # The optimized pattern requires QNN 2.48+. Resolving the SDK version
+        # relies on QNN_SDK_ROOT; when it is unavailable (e.g. AOT-only
+        # environments) fall back to the legacy lowering, which is valid on all
+        # QNN versions.
+        if not os.environ.get("QNN_SDK_ROOT") or is_qnn_sdk_version_less_than("2.48"):
             return self.define_node_legacy(node, nodes_to_wrappers)
         return self.define_node_optimize(node, nodes_to_wrappers)

backends/qualcomm/utils/check_qnn_version.py

@@ -26,8 +26,13 @@ def _get_qnn_host_lib_dir_name() -> str:
 
 
 def get_sdk_build_id():
+    qnn_sdk_root = os.environ.get("QNN_SDK_ROOT")
+    if not qnn_sdk_root:
+        raise EnvironmentError(
+            "QNN_SDK_ROOT must be set to query the QNN SDK build id."
+        )
     htp_library_path = os.path.join(
-        os.environ.get("QNN_SDK_ROOT", None),
+        qnn_sdk_root,
         "lib",
         _get_qnn_host_lib_dir_name(),
         get_qnn_lib_name("QnnHtp"),