Use async iteration for progressive outputs

[freddyaboulton]

Description

I noticed that if two different users are running an iterative output demo at the same time, the two demos will not iterate asynchronously. When one demo is iterating, the other is not. This increases the latency of serving outputs to users.

You can see this here:

Main

This branch

The culprit is that next(iterator) is not async, so it blocks the event loop for all users.

This is the demo code to test:

import time
import gradio as gr
import random

def fake_diffusion(steps):
    for i in range(steps):
        time.sleep(1)
        yield random.choice([
            "https://images.unsplash.com/photo-1507003211169-0a1dd7228f2d?ixlib=rb-1.2.1&ixid=MnwxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8&auto=format&fit=crop&w=387&q=80",
            "https://images.unsplash.com/photo-1554151228-14d9def656e4?ixlib=rb-1.2.1&ixid=MnwxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8&auto=format&fit=crop&w=386&q=80",
            "https://images.unsplash.com/photo-1542909168-82c3e7fdca5c?ixlib=rb-1.2.1&ixid=MnwxMjA3fDB8MHxzZWFyY2h8MXx8aHVtYW4lMjBmYWNlfGVufDB8fDB8fA%3D%3D&w=1000&q=80",
            "https://images.unsplash.com/photo-1546456073-92b9f0a8d413?ixlib=rb-1.2.1&ixid=MnwxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8&auto=format&fit=crop&w=387&q=80",
            "https://images.unsplash.com/photo-1601412436009-d964bd02edbc?ixlib=rb-1.2.1&ixid=MnwxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8&auto=format&fit=crop&w=464&q=80",
            ])

with gr.Blocks() as demo:
    n = gr.Slider(1, 10, value=3, step=1)
    output = gr.Image().style(height=250, width=250)
    run = gr.Button()
    run.click(fake_diffusion, n, output)

demo.queue(concurrency_count=20, max_size=20).launch()

Checklist:

• I have performed a self-review of my own code
• I have added a short summary of my change to the CHANGELOG.md
• My code follows the style guidelines of this project
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

A note about the CHANGELOG

Hello 👋 and thank you for contributing to Gradio!

All pull requests must update the change log located in CHANGELOG.md, unless the pull request is labeled with the "no-changelog-update" label.

Please add a brief summary of the change to the Upcoming Release > Full Changelog section of the CHANGELOG.md file and include
a link to the PR (formatted in markdown) and a link to your github profile (if you like). For example, "* Added a cool new feature by [@myusername](link-to-your-github-profile) in [PR 11111](https://github.com/gradio-app/gradio/pull/11111)".

If you would like to elaborate on your change further, feel free to include a longer explanation in the other sections.
If you would like an image/gif/video showcasing your feature, it may be best to edit the CHANGELOG file using the
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[github-actions]

All the demos for this PR have been deployed at https://huggingface.co/spaces/gradio-pr-deploys/pr-2409-all-demos

[abidlabs]

Great catch @freddyaboulton! Will review shortly

[abidlabs]

Would StopAsyncIteration work here? https://stackoverflow.com/questions/50165734/native-coroutine-and-stopiteration

[abidlabs]

LGTM @freddyaboulton reproduced the original issue and confirmed that this fixes it. Just a comment above in case we can avoid defining a new exception.

[freddyaboulton]

Thanks for the review and suggestion @abidlabs ! StopAsyncIteration works!