Hierarchical CP implementation (Ulysses + Ring)

[xrennvidia]

Description

This PR adds a hierarchical implementation of context parallelism to attention. It uses A2A communications in low-level CP groups (e.g., via NVLink), and P2P communications in high-level CP groups (e.g., via IBLink). For more details, please refer to LongVILA and USP.

This implementation supports:

• backends: FusedAttention, FlashAttention
• dtype: BF16/FP16 (both backends), FP8 (FusedAttention only)
• mask_type: causal, no_mask
• attention types: MHA, MQA/GQA
• qkv_format: sbhd, bshd
• bias type: no_bias

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)
• [x] 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 refractor

Checklist:

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