Fix libc++.so.1 missing for qnn-context-binary-utility (#19622)

[psiddh]

Summary:

The QNN SDK 2.37 prebuilt 'qnn-context-binary-utility' is dynamically
linked against libc++.so.1, which is shipped at $QNN_SDK_ROOT/lib//
but not visible on the default LD_LIBRARY_PATH inside the fbcode test
sandbox. As a result, every invocation of validate_context_binary failed
with:

qnn-context-binary-utility: error while loading shared libraries:
libc++.so.1: cannot open shared object file: No such file or directory

…and the test asserted on a missing ctx.json with a useless
'AssertionError: None' (because print() returns None).

This change adds a small _qnn_subprocess_env() helper that prepends the
QNN SDK's bundled lib dir to LD_LIBRARY_PATH and wires it into both
validate_context_binary and generate_context_binary subprocess calls.
Also replaces print(result.stderr) inside the assert with a proper
diagnostic message including stdout and stderr.

Affected tests (previously 0% pass rate, now passing):

• test_qnn_backend_dump_context_from_pte (Float + Quant variants)
• test_qnn_backend_context_extraction (Float + Quant variants)

Differential Revision: D105378870

[pytorch-bot]

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@psiddh has exported this pull request. If you are a Meta employee, you can view the originating Diff in D105378870.

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[Copilot]

Pull request overview

This PR fixes Qualcomm QNN test failures caused by QNN SDK prebuilt tools (notably qnn-context-binary-utility in QNN SDK 2.37) being unable to locate their bundled libc++.so.1 inside the test sandbox. It does so by explicitly prepending the QNN SDK’s lib/<target>/ directory to LD_LIBRARY_PATH for subprocess invocations, and improves failure diagnostics for missing ctx.json.

Changes:

• Add _qnn_subprocess_env() helper to construct an env dict with LD_LIBRARY_PATH including $QNN_SDK_ROOT/lib/<target>.
• Wire the helper into subprocess calls for context binary generation/validation.
• Improve the validate_context_binary assertion message to include stdout/stderr when ctx.json is not produced.

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[Copilot]

Pull request overview

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