[Misc] Make gradio an optional dependency and upgrade to >=6.7.0 (#2221)

Signed-off-by: Lidang-Jiang <lidangjiang@gmail.com>
Signed-off-by: Lidang Jiang <lidangjiang@gmail.com>

docs/getting_started/installation/gpu/cuda.inc.md

@@ -29,6 +29,11 @@ uv pip install vllm --torch-backend=auto
 uv pip install vllm-omni
 ```
 
+To run Gradio demos, also install the optional extras:
+```bash
+uv pip install 'vllm-omni[demo]'
+```
+
 # --8<-- [end:pre-built-wheels]
 
 # --8<-- [start:build-wheel-from-source]
@@ -50,6 +55,11 @@ cd vllm-omni
 uv pip install -e .
 ```
 
+To run Gradio demos, install with optional extras:
+```bash
+uv pip install -e '.[demo]'
+```
+
 <details><summary>(Optional) Installation of vLLM from source</summary>
 If you want to check, modify or debug with source code of vLLM, install the library from source with the following instructions: