Enable THD with hierarchical context parallelism

[sudhakarsingh27]

Description

The THD-aware A2A and P2P paths already compose correctly and match the non-CP reference in forward and backward passes. Remove the stale runtime and backend gates, and enable the existing FlashAttention and FusedAttention test coverage for this combination.
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

Please list the changes introduced in this PR:

Only removes filters which are no longer necessary

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