feat(diffusion): select Wan VAE decode mode automatically

Signed-off-by: Hongsheng Liu <liuhongsheng4@huawei.com>

docs/design/feature/vae_parallel.md

@@ -451,16 +451,16 @@ Complete examples in the codebase:
 
 ## Spatially-Sharded Decode (Wan)
 
-The tile-parallel executor above assigns independent spatial **tiles** to ranks. The Wan VAE additionally supports a **spatially-sharded decode** backend, selected through `DiffusionParallelConfig.vae_parallel_mode` (`"spatial_shard_height"` or `"spatial_shard_width"`; default is `"tile"`).
+The tile-parallel executor above assigns independent spatial **tiles** to ranks. The Wan VAE additionally supports a **spatially-sharded decode** backend. `DiffusionParallelConfig.vae_parallel_mode="auto"` is the default; explicit `"tile"`, `"spatial_shard_height"`, and `"spatial_shard_width"` modes remain available.
 
 ### How it differs from tile parallel
 
-| Aspect | Tile parallel (`"tile"`) | Spatially-sharded (`"spatial_shard_height"`/`"spatial_shard_width"`) |
-|--------|--------------------------|-----------------------------------------------|
-| Unit of work | Independent overlapping tiles | A single global feature map sharded along H or W |
-| Cross-rank communication | Gather tiles to rank 0, stitch + blend | Per-conv **halo exchange** of boundary rows/cols (P2P) |
-| Output assembly | Blend overlapping tiles | All-gather shards on rank 0, trim padding (matches `broadcast_result=False`) |
-| Scope | Decode + encode | Decode only |
+| Aspect | Tile parallel (`"tile"`) | Automatic (`"auto"`) | Spatially-sharded (`"spatial_shard_height"`/`"spatial_shard_width"`) |
+|--------|--------------------------|----------------------|-----------------------------------------------|
+| Unit of work | Independent overlapping tiles | Chooses tile or a spatial shard per request | A single global feature map sharded along H or W |
+| Cross-rank communication | Gather tiles to rank 0, stitch + blend | Depends on the selected path | Per-conv **halo exchange** of boundary rows/cols (P2P) |
+| Output assembly | Blend overlapping tiles | Depends on the selected path | All-gather shards on rank 0, trim padding (matches `broadcast_result=False`) |
+| Scope | Decode + encode | Wan decode selection; encode remains tiled | Decode only |
 
 Spatial-shard decode swaps the decoder's spatial convolutions/padding for halo-exchanging variants (`WanDistConv2d`, `WanDistCausalConv3d`, `WanDistZeroPad2d`) so each rank only holds a shard of the activations but still sees the correct receptive field at shard boundaries. Implementation lives in `vllm_omni/diffusion/distributed/autoencoders/wan_spatial_shard.py`.
 
@@ -476,11 +476,11 @@ serve.py (--vae-parallel-mode) / OmniEngineArgs
   -> DistributedAutoencoderKLWan.tiled_decode dispatch
 ```
 
-`DistributedAutoencoderKLWan._spatial_shard_decode_enabled()` gates the path: it requires distributed decode to be enabled, a 5D latent, and `vae_patch_parallel_size == DiT group size`. Otherwise it logs a warning and falls back to tile-parallel decode.
+`DistributedAutoencoderKLWan._spatial_shard_decode_enabled()` gates the path: it requires distributed decode to be enabled, a 5D latent, and `vae_patch_parallel_size == DiT group size`. The method is reached only after Diffusers has selected tiled decode for the request. In `auto` mode the longer axis that can cover every rank is selected, with width winning ties. This policy is cached only through normal kernel shape caches; it is reevaluated for every request.
 
 ### Notes
 
-- The decoder is patched **in place** the first time spatial-shard decode runs and is bound to a single split dimension for the lifetime of the VAE instance.
+- The decoder is patched **in place** the first time spatial-shard decode runs. Its wrappers consult a context-local split dimension and retain an exact direct fallback, allowing later requests to select tile, height, or width safely.
 - Numerical correctness vs. single-GPU decode is covered by `tests/diffusion/distributed/test_wan_spatial_shard.py::test_spatial_shard_decode_matches_reference` (multi-GPU, nightly `full_model` + `distributed_cuda`).
 
 ---

docs/user_guide/diffusion/parallelism/vae_parallelism.md

@@ -113,7 +113,7 @@ In `DiffusionParallelConfig`:
 | Parameter | Type | Default | Description |
 |-----------|------|---------|-------------|
 | `vae_patch_parallel_size` | int | 1 | Number of GPUs for VAE patch/tile parallelism. Set to 2 or higher to enable. Should typically match `tensor_parallel_size` as they share the same process group. |
-| `vae_parallel_mode` | str | `"tile"` | VAE parallel decode strategy: `"tile"` (default tile/patch parallel decode), `"spatial_shard_height"`, or `"spatial_shard_width"` (spatially-sharded decode, Wan only). See [Spatially-Sharded Decode](#spatially-sharded-decode-wan). |
+| `vae_parallel_mode` | str | `"auto"` | VAE parallel decode strategy: `"auto"` (shape/model/topology-aware selection), `"tile"`, `"spatial_shard_height"`, or `"spatial_shard_width"`. Spatial sharding is currently Wan-only. See [Spatially-Sharded Decode](#spatially-sharded-decode-wan). |
 
 Additional requirements:
 
@@ -128,7 +128,7 @@ Additional requirements:
 
 ## Spatially-Sharded Decode (Wan)
 
-The default `vae_parallel_mode="tile"` distributes whole tiles across ranks. For the **Wan** VAE there is an alternative decode strategy, **spatially-sharded decode**, selected via `vae_parallel_mode="spatial_shard_height"` or `vae_parallel_mode="spatial_shard_width"`.
+The default `vae_parallel_mode="auto"` selects spatial-shard decode only after a Wan request is already large enough to enter Diffusers' tiled-decode path and the VAE uses the full DiT decode group. It splits the longer eligible spatial axis to reduce halo traffic. Direct-decode requests, partial groups, and other VAE families keep their existing path. Set `vae_parallel_mode="tile"` to force the previous behavior, or `"spatial_shard_height"` / `"spatial_shard_width"` to force an axis.
 
 Instead of assigning independent tiles to ranks, spatial-shard decode shards the decoder feature maps along the height (`spatial_shard_height`) or width (`spatial_shard_width`) dimension and exchanges halo rows/columns between neighboring ranks around the spatial convolutions. This keeps the receptive field correct across shard boundaries, so the result matches the single-GPU decode within numerical tolerance.
 
@@ -141,7 +141,7 @@ omni = Omni(
     parallel_config=DiffusionParallelConfig(
         tensor_parallel_size=2,
         vae_patch_parallel_size=2,               # must match the DiT group size
-        vae_parallel_mode="spatial_shard_width", # or "spatial_shard_height"
+        vae_parallel_mode="auto",                # default
     ),
 )
 ```
@@ -152,14 +152,16 @@ Or from the CLI / serving entrypoint:
 vllm serve Wan-AI/Wan2.1-T2V-1.3B-Diffusers --omni \
     --tensor-parallel-size 2 \
     --vae-patch-parallel-size 2 \
-    --vae-parallel-mode spatial_shard_width
+    --vae-parallel-mode auto
 ```
 
 **Constraints and behavior:**
 
 - Spatial-shard decode is **decode-only** and currently implemented for the **Wan** VAE. Other models ignore `spatial_shard_*` modes.
-- It requires `vae_patch_parallel_size` to **match the DiT process group size**. If it does not, the VAE logs a warning and **falls back to tile-parallel decode** at runtime.
-- `spatial_shard_height` and `spatial_shard_width` are mutually exclusive for a given VAE instance (the decoder is patched in place for a single split dimension).
+- It requires `vae_patch_parallel_size` to **match the DiT process group size**. `auto` quietly falls back to tile decode for a partial group; an explicitly forced spatial mode logs a warning.
+- The decoder's context-aware wrappers retain the direct path, so one process can alternate small/large and portrait/landscape requests safely.
+
+The automatic policy is adapted from [SGLang #28071](https://github.com/sgl-project/sglang/pull/28071) and narrowed to vLLM-Omni's parity-tested Wan backend.
 
 For end-to-end latency/throughput, launch serving with the desired `vae_parallel_mode` and use the existing diffusion serving benchmark: