Fix embedding lowering crash when QNN_SDK_ROOT is unset

[psiddh]

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

[pytorch-bot]

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[meta-codesync]

@psiddh has exported this pull request. If you are a Meta employee, you can view the originating Diff in D112944232.

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[shewu-quic]

LGTM. Thanks!

[Copilot]

Pull request overview

This PR fixes a crash in Qualcomm QNN embedding lowering during AOT partitioning when QNN_SDK_ROOT is unset, by preventing SDK-version probing in environments that don’t have an SDK path configured and by improving the error raised when the build-id helper is called without the required environment variable.

Changes:

• Make get_sdk_build_id() explicitly raise EnvironmentError with a clear message when QNN_SDK_ROOT is missing.
• Update embedding lowering to fall back to the legacy lowering when QNN_SDK_ROOT is unset, and only attempt the QNN 2.48+ optimized lowering when the SDK root is available.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
backends/qualcomm/utils/check_qnn_version.py	Adds an explicit environment-variable check and clearer exception for SDK build-id lookup.
backends/qualcomm/builders/op_embedding.py	Avoids version-gated optimized embedding lowering when SDK root is unavailable to prevent AOT partitioning crashes.

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