[Core] Support online FP8 with DLO AllGather (#6279)

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

docs/user_guide/diffusion/offloader/distributed_layerwise_offload.md

@@ -138,8 +138,11 @@ must enter each collective.
   but it provides no shared-mmap host-memory benefit or guarantee.
 - HSDP plus AllGather is rejected to avoid double sharding. HSDP without
   AllGather has limited end-to-end validation.
-- Online quantization uses the ordinary loader with no-AllGather. It remains
-  incompatible with DLO AllGather.
+- Per-tensor online FP8 linears use the ordinary loader and can run with either
+  DLO transfer path. With AllGather, every rank temporarily materializes the
+  complete FP8 model in host memory before DLO retains only its shard. Other
+  online quantization methods require no-AllGather until their runtime layouts
+  are validated.
 - Resident leading layers require `--dlo-no-use-allgather` and a model
   `OffloadPlan` that declares eligible `resident_dit_paths`.
 - DP concurrency requires an explicit, identical inference-step count.

docs/user_guide/quantization/fp8.md

@@ -179,6 +179,24 @@ python text_to_image.py --model <your-model> --quantization fp8 --ignored-layers
 vllm serve <your-model> --omni --quantization fp8
 ```
 
+### Distributed layerwise offload
+
+Per-tensor online FP8 linears can be combined with DLO's default AllGather path:
+
+```bash
+vllm serve <your-model> \
+  --omni \
+  --quantization fp8 \
+  --enable-distributed-layerwise-offload
+```
+
+Direct checkpoint mmap is not used for this combination. Each rank runs the
+ordinary loader to create the final FP8 weights and scales before DLO shards
+them, so startup temporarily materializes the complete FP8 model in host memory
+on every rank. Runtime DLO host residency is sharded normally. Use
+`--dlo-no-use-allgather` when the synchronized AllGather request-wave contract
+does not fit the workload.
+
 ## Parameters
 
 | Parameter | Type | Default | Description |