fix API thread safe issues of txt2img and img2img

[lanyeeee]

Description

TL;DR:

Function ui.create_ui and self.init_default_script_args are not thread-safe.
It causes thread-safety issues of API txt2img and img2img.
To ensure the API is thread-safe, I move the code to init

Steps to reproduce the thread-safety issues

1. Run webui with the following command

.\webui.bat --api --disable-all-extensions --nowebui

2. Run the following python script

import aiohttp
import asyncio


async def main():
    async with aiohttp.ClientSession() as session:
        task1 = session.request('POST', url='http://127.0.0.1:7861/sdapi/v1/txt2img', json={})
        task2 = session.request('POST', url='http://127.0.0.1:7861/sdapi/v1/txt2img', json={})
        task3 = session.request('POST', url='http://127.0.0.1:7861/sdapi/v1/txt2img', json={})
        await asyncio.gather(task1, task2, task3, return_exceptions=True)


asyncio.run(main())

In most cases you will see the error on the terminal which running webui.
If not encounter the error, ctrl+c cancel the webui then repeat the above steps again until the error appears

Fix issue #10884

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests

[AUTOMATIC1111]

I think there might be some issues if you use same scripts.scripts_img2img for both txt2img and img2img. Also, does this impact startup time?

[lanyeeee]

Also, does this impact startup time?

I don't think it will cause a noticeable impact on startup time.

But to make sure it doesn't impact startup time.
I give up move the code to __init__, instead use locks

[AUTOMATIC1111]

Actually, I'm sorry to say it now, but I liked the original version without locks a lot more. If there's no noticeable impact, I'd rather have that, just with two script_runners instead of one.