[perf] Add configurable diffusion compile granularity (#4603)

Signed-off-by: TaffyOfficial <2324465096@qq.com>
Co-authored-by: TaffyOfficial <2324465096@qq.com>

docs/user_guide/diffusion/regional_compilation.md

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+# Regional Compilation
+
+Regional compilation applies `torch.compile` to the repeated transformer blocks
+declared by a diffusion model. It is the default compilation scope when
+diffusion inference runs without `--enforce-eager`.
+
+## Configuration
+
+Dynamic compilation is enabled by default so the compiled regions can handle
+mixed resolutions. For a fixed-shape workload, disable it explicitly:
+
+```bash
+vllm serve <model> --omni --no-diffusion-compile-dynamic
+```
+
+The equivalent per-stage deploy configuration is:
+
+```yaml
+stages:
+  - stage_id: 0
+    diffusion_compile_granularity: regional
+    diffusion_compile_dynamic: false
+```
+
+For an experimental whole-transformer compile scope, set
+`--diffusion-compile-granularity full` or use
+`diffusion_compile_granularity: full` in the stage configuration. Full scope may
+still contain graph breaks; it does not force one graph. It is rejected when
+HSDP, sequence parallelism, CPU offload, or layerwise offload is enabled. Use
+regional scope with those features.
+
+These settings control the generic model-runner compilation path. Pipelines
+that provide their own `setup_compile()` implementation manage their compilation
+policy independently. Compilation is lazy, so backend or graph errors can first
+surface on the initial request.
+
+Use `--enforce-eager` to disable the model runner's generic compile setup.
+Pipelines that compile internally define their own eager-mode behavior.