Support base_url in OpenAI module

[kazuhitoyokoi]

The OpenAI module already has functionality to access other APIs. This pull request allows the base_url setting to specify other APIs. For example, by supporting this setting, we can access Google Gemini API or Ollama API using the following code.

Google Gemini

import gradio as gr
import openai_gradio

gr.load(
    name='gemini-2.0-flash-exp',
    token='<Gemini API Key>',
    base_url="https://generativelanguage.googleapis.com/v1beta/",
    src=openai_gradio.registry,
).launch()

Ollama

import gradio as gr
import openai_gradio

gr.load(
    name='llama3.2',
    token='ollama',
    base_url="http://localhost:11434/v1",
    src=openai_gradio.registry,
).launch()