Fix and harden attention benchmark script

[KshitijLakhani]

• Add project root to sys.path so tests can be imported from any cwd.
• Fix ModelConfig args: use correct order and keywords.
• Run benchmark in main process first (append_csv=True) so times.csv is always populated; nsys subprocess uses append_csv=False. Fix subprocess -c to a single string and pass cwd.
• Unpack three return values from _run_dot_product_attention (out, max_logit, grads).
• Show module timings (FusedAttention/FlashAttention Module) in the summary; display "N/A" for speedup when only one backend ran.
• Harden parse_results: configurable kernel name constants, case-insensitive match, skip if trace file missing.

Description

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Fixes # (issue)

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Changes

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• Change A
• Change B

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes