[Diffusion][WIP]: Diffusion Ring Attention support

[mxuax]

This PR follows Diffusion Ulysses-Sequence-Parallelism support PR#189, allowing users to set Ring Attention for diffusion model, e.g., qwen-image.

Purpose

1. Create Ring Attention Module (vllm_omni/diffusion/ring_attn)

2. Refactor Distributed State (vllm_omni/diffusion/distributed/parallel_state.py)

3. Implement Ring Attention Logic (vllm_omni/diffusion/attention/layer.py)

4. Modify the generic Attention layer to support Ring Attention and Hybrid (Ulysses + Ring) modes.

5. Verification and Testing
   Create a new test file to verify the correctness of the Ring Attention implementation on the Qwen model components.

Test Plan

• UT: Test File: tests/distributed/test_ring_attention.py

Test Result

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