sdpa: throw cudnnGraphNotSupportedException when no implementation supports the attributes

[vedaanta]

What

When _auto_select_implementation finds no implementation for the requested SDPA feature combination on the current arch/version, sdpa_internal threw a plain std::runtime_error. Throw cudnnGraphNotSupportedException instead - it subclasses std::runtime_error (existing catch sites keep working) and pybind already maps it to cudnn.cudnnGraphNotSupportedError.

Why

test_sdpa_fp8_fwd_ragged_L0 draws cu_ragged/cu_ragged_mult layouts. On SM90 fp8 there is no implementation for CU_SEQ_LEN inputs (the composite path rejects them by design, the unified engines do not cover SM90 fp8), which is a legitimate unsupported configuration - but the generic RuntimeError made the test framework pytest.fail instead of skip, so the suite reported 23 hard failures on H100. C++ callers likewise could not distinguish unsupported-graph from a real error.

Verification (H100, cuDNN dev build)

-k fp8_fwd_ragged: before = 23 failed / 2 passed / 7 skipped; after = 2 passed / 30 skipped / 0 failed. Numeric failures are unaffected - only the no-implementation path changes type.

Note: branch is based on 6c39f8b rather than develop tip because tip (#612) currently has a broken import - _pygraph.py imports ensure_current_context from cudnn._device, which no longer defines it, so import cudnn fails. Flagging separately.

Summary by CodeRabbit

• Bug Fixes
  • Improved error reporting for unsupported SDPA configurations by providing a more specific graph support exception.

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 218eee7 and 96e0cc8.

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• include/cudnn_frontend/graph_interface.h

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📝 Walkthrough

Walkthrough

The SDPA internal dispatch path now throws cudnnGraphNotSupportedException when no implementation supports the requested configuration.

Changes

SDPA dispatch

Layer / File(s)	Summary
Typed unsupported-configuration error include/cudnn_frontend/graph_interface.h	The unsupported SDPA path now throws cudnnGraphNotSupportedException instead of std::runtime_error.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: ⚪ Minimal · up to 96e0c

The change classifies unsupported SDPA configurations as a supported-not-available error so callers and tests can handle them appropriately; no actionable merge-blocking risk remains after normal checks and review.

Suggested reviewers: anerudhan

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the SDPA exception change when no implementation supports the requested attributes.
Description check	✅ Passed	The description clearly explains the change, motivation, compatibility impact, affected tests, and verification results, despite not following every template heading.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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