Enable FA4 for context-parallel attention

[sudhakarsingh27]

Description

Enable FlashAttention 4 for context-parallel attention on the already-supported CP communication shapes: p2p, all_gather, and a2a.

FA4 can represent padded THD layouts with physical padded offsets plus seqused_* visible lengths. This lets the CP path handle fa_pad_between_seqs=True with the same layout distinction used by the non-CP reference path. The hierarchical a2a+p2p path remains disabled because it has not been validated for FA4.

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

• Route FA4 raw forward/backward calls through the CP p2p, all_gather, and a2a FlashAttention paths.
• Pass padded THD cu_seqlens_* together with seqused_* for FA4 when padding exists between sequences.
• Keep FA4 disabled for a2a+p2p until that path is separately validated.
• Update FlashAttention CP tests so FA4-capable environments exercise the CP matrix and padded THD guards.

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

Validation

• git diff --check
• python3.12 -m py_compile on the touched attention/test files
• Static forward/backward arity check for the three touched CP autograd functions
• Padded THD FlashAttention CP pytest subset on H100-class and B200-class GPUs: 6 passed, 10 skipped, 48 deselected on each platform. The skips are existing unsupported combinations: p2p+SWA, MLA with non-P2P CP, and padded THD with a2a+p2p.