[Doc] update offline inference doc and offline_inference examples (#274)

Signed-off-by: David Chen <530634352@qq.com>

docs/user_guide/examples/offline_inference/text_to_image.md

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+# Qwen-Image Offline Inference
+
+Source <https://github.com/vllm-project/vllm-omni/tree/main/examples/offline_inference/text_to_image>.
+
+
+This folder provides two simple entrypoints for experimenting with `Qwen/Qwen-Image` `Tongyi-MAI/Z-Image-Turbo` using vLLM-Omni:
+
+- `text_to_image.py`: command-line script for single image generation.
+- `web_demo.py`: lightweight Gradio UI for interactive prompt/seed/CFG exploration.
+
+
+## Local CLI Usage
+
+```bash
+python text_to_image.py \
+  --model Tongyi-MAI/Z-Image-Turbo \
+  --prompt "a cup of coffee on the table" \
+  --seed 42 \
+  --cfg_scale 4.0 \
+  --num_images_per_prompt 1 \
+  --num_inference_steps 50 \
+  --height 1024 \
+  --width 1024 \
+  --output outputs/coffee.png
+```
+
+Key arguments:
+
+- `--prompt`: text description (string).
+- `--seed`: integer seed for deterministic sampling.
+- `--cfg_scale`: true CFG scale (model-specific guidance strength).
+- `--num_images_per_prompt`: number of images to generate per prompt (saves as `output`, `output_1`, ...).
+- `--num_inference_steps`: diffusion sampling steps (more steps = higher quality, slower).
+- `--height/--width`: output resolution (defaults 1024x1024).
+- `--output`: path to save the generated PNG.
+
+## Web UI Demo
+
+Launch the gradio demo:
+
+```bash
+python gradio_demo.py --port 7862
+```
+
+Then open `http://localhost:7862/` on your local browser to interact with the web UI.
+
+## Example materials
+
+??? abstract "gradio_demo.py"
+    ``````py
+    --8<-- "examples/offline_inference/text_to_image/gradio_demo.py"
+    ``````
+??? abstract "text_to_image.py"
+    ``````py
+    --8<-- "examples/offline_inference/text_to_image/text_to_image.py"
+    ``````