[diffusion] fix: keep Qwen-Image 2.1 prefix KV per layer under Cache-DiT (#40472)

docs/cookbook/diffusion/Qwen-Image/Qwen-Image-2.1.mdx

@@ -216,3 +216,11 @@ Keep eager execution as the default. Breakable CUDA Graph replay requires
 matching resolution and condition-prefix length; unseen shapes run eagerly.
 Text buckets alone do not guarantee replay. SageAttention and Cache-DiT can
 change numerical results and require quality checks for your workload.
+
+### Cache-DiT
+
+Enable `--enable-cache-dit true` or `SGLANG_CACHE_DIT_ENABLED=true`. 2.1 prefix
+KV is per layer: each block slices caches by `_layer_id`. Cache-DiT wraps
+`transformer_blocks` and forwards the same extras to every layer; without that
+slice, later layers reuse layer 0 and the image collapses to color noise.
+See the [Cache-DiT guide](/docs/sglang-diffusion/cache_dit).

docs/docs/sglang-diffusion/cache_dit.mdx

@@ -783,7 +783,7 @@ SGLang Diffusion x Cache-DiT supports almost all models originally supported in
     </tr>
     <tr>
       <td style={{padding: "9px 12px", fontWeight: 500, backgroundColor: "rgba(255,255,255,0.02)"}}>Qwen</td>
-      <td style={{padding: "9px 12px", backgroundColor: "rgba(255,255,255,0.05)"}}>Qwen-Image, Qwen-Image-Edit</td>
+      <td style={{padding: "9px 12px", backgroundColor: "rgba(255,255,255,0.05)"}}>Qwen-Image, Qwen-Image-Edit, Qwen-Image 2.1</td>
     </tr>
     <tr>
       <td style={{padding: "9px 12px", fontWeight: 500, backgroundColor: "rgba(255,255,255,0.02)"}}>Hunyuan</td>