[Docs] Refactor the attention backend docs/skill (#3475)

Signed-off-by: gcanlin <canlinguosdu@gmail.com>
Co-authored-by: Hongsheng Liu <liuhongsheng4@huawei.com>

docs/contributing/model/adding_diffusion_model.md

@@ -115,17 +115,20 @@ class YourAttentionBlock(nn.Module):
 from vllm_omni.diffusion.attention.layer import Attention
 from vllm_omni.diffusion.attention.backends.abstract import AttentionMetadata
 
-class YourAttentionBlock(nn.Module):
+class YourSelfAttentionBlock(nn.Module):
     def __init__(self, ...):
         super().__init__()
 
-        # Initialize vLLM-Omni's Attention layer
+        # Initialize vLLM-Omni's Attention layer.
+        # `role` lets users target this site with --diffusion-attention-config
+        # (e.g. --diffusion-attention-config.per_role.self.backend SAGE_ATTN).
         self.attn = Attention(
             num_heads=self.num_heads,
             head_size=self.head_dim,
             softmax_scale=1.0 / (self.head_dim ** 0.5),
             causal=False,  # Diffusion models typically use bidirectional attention
             num_kv_heads=self.num_kv_heads,
+            role="self",
         )
 
     def forward(self, hidden_states, encoder_hidden_states=None, attention_mask=None, ...):
@@ -133,7 +136,6 @@ class YourAttentionBlock(nn.Module):
         # Create attention metadata
         attn_metadata = AttentionMetadata(attn_mask=attention_mask)
         hidden_states = self.attn(query, key, value, attn_metadata=attn_metadata)
-
 ```
 
 **Key Points:**
@@ -141,8 +143,35 @@ class YourAttentionBlock(nn.Module):
 - **Attention layer initialization:** Done in `__init__`, not per-forward
 - **Tensor shapes:** vLLM-Omni `Attention` expects QKV to have `[B, seq, num_heads, head_dim]` shape
 - **AttentionMetadata:** Wraps attention mask and other metadata
+- **Role:** Tag every `Attention` site with a `role` string so users can configure backends per role (see below)
 
-**Attention backends:** vLLM-Omni automatically selects the attention backend given the environmental variable `DIFFUSION_ATTENTION_BACKEND`. The default attention backend is `FLASH_ATTN` for diffusion models.
+**Declaring attention roles**
+
+The `role` argument is a free-form string that identifies this attention site. Users can match it from `--diffusion-attention-config.per_role.<role>.*` to swap backends without touching model code. Two conventions cover the common cases:
+
+| Convention | When to use | Example |
+|---|---|---|
+| `"self"` | Q/K/V come from the same hidden state | DiT self-attention block |
+| `"cross"` | K/V come from a separate `encoder_hidden_states` | Text-conditioned cross-attention |
+
+For multi-modal or unusual sites, use a dot-namespaced role and pair it with `role_category` so it can fall back to the generic config when nothing model-specific is set:
+
+```python
+# A model-specific cross-attention site that user config can target
+# either as 'mymodel.audio_to_video' (exact) or as 'cross' (category fallback).
+self.audio_to_video_attn = Attention(
+    num_heads=self.num_heads,
+    head_size=self.head_dim,
+    softmax_scale=1.0 / (self.head_dim ** 0.5),
+    causal=False,
+    role="mymodel.audio_to_video",
+    role_category="cross",
+)
+```
+
+For cross-attention sites whose K/V are replicated across ranks (e.g. text encoder output), pass `skip_sequence_parallel=True` to opt this layer out of sequence-parallel sharding.
+
+**Attention backends:** When the user does not configure a backend, vLLM-Omni asks the current platform for its default (typically `FLASH_ATTN` on CUDA when available). Users override the default via `--diffusion-attention-backend`, the `DIFFUSION_ATTENTION_BACKEND` env var, or finer-grained `--diffusion-attention-config.per_role.*` flags. See [Diffusion Attention Backends](../../user_guide/diffusion/attention_backends.md) for the full configuration surface.
 
 #### 1.3: Replace Imports and Utilities