[diffusion][feature] Add offline SVDQuant W4A4 support (#6162)

Signed-off-by: lishunyang12 <lishunyang12@163.com>
Co-authored-by: Hongsheng Liu <liuhongsheng4@huawei.com>

docs/user_guide/quantization/svdquant.md

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+# SVDQuant W4A4
+
+## Overview
+
+[SVDQuant](https://arxiv.org/abs/2411.05007) combines four-bit weights
+and activations with a small low-rank branch that corrects part of the
+quantization error. vLLM-Omni consumes an offline-quantized checkpoint;
+it does not calibrate the model while loading.
+
+## Checkpoint contract
+
+Place the following entry in the diffusion transformer's `config.json`:
+
+```json
+{
+  "quantization_config": {
+    "quant_method": "svdquant",
+    "rank": 32,
+    "precision": "nvfp4",
+    "act_unsigned": false,
+    "modules_to_not_convert": []
+  }
+}
+```
+
+For a quantized linear with input size `K`, output size `N`, and correction
+rank `R`, the checkpoint stores:
+
+| Suffix | Shape | dtype |
+| --- | --- | --- |
+| `qweight` | `(N, K / 2)` | `int8` (two packed FP4 values per byte) |
+| `wscales` | `(K / 16, N)` | `float8_e4m3fn` |
+| `proj_down` | `(K, R)` | `bfloat16` |
+| `proj_up` | `(N, R)` | `bfloat16` |
+| `smooth_factor` | `(K,)` | `bfloat16` |
+| `wcscales` | `(N,)` | `bfloat16` |
+| `wtscale` | `(1,)` | `bfloat16` |
+
+`K` must be divisible by 16 on every tensor-parallel rank. Set `wcscales` to
+ones when no per-output correction is needed. Modules listed in
+`modules_to_not_convert` keep their checkpoint precision.
+
+## Runtime support
+
+The compatibility path accepts BF16 inputs and executes an NVFP4 GEMM followed
+by the BF16 rank correction. It supports vLLM's FlashInfer, CUTLASS, and FBGEMM
+NVFP4 tensor layouts; incompatible forced backends fail during model loading.
+SM103 is the currently validated and enabled hardware target. Native fusion of
+the NVFP4 GEMM and rank correction is separate from this checkpoint-loading
+contract.