docs: fix standalone test script working directory in tests/README.md
[resolve #2] Review comment by @Copilot (PR #21614):
"These commands run run_standalone_tasks.sh from the tests/ directory, but the script
explicitly assumes it is run with the working directory tests/tests_pytorch and uses
relative paths that will fail otherwise."
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Co-authored-by: Claude Code <noreply@anthropic.com>