Add option to display gallery in preview mode

[freddyaboulton]

Description

Fixes #3311

• If style=True and the gallery is in a "Cleared State", i.e. value is [] or None, then the next update should display the gallery in preview mode.
• Otherwise, the behavior should be the same compared to main

To test,

import random

import gradio as gr
from PIL import Image


def create_images(n_images):

    a = Image.new("RGB", (512, 512), "red")
    b = Image.new("RGB", (512, 512), "green")
    c = Image.new("RGB", (512, 512), "blue")

    res = [a, b, c][:n_images]
    random.shuffle(res)
    return res

gr.Interface(create_images,
             inputs=[gr.Slider(value=3, minimum=1, maximum=3, step=1)],
             outputs=[gr.Gallery().style(grid=2, preview=True)]).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
GitHub web UI since that lets you upload files directly via drag-and-drop.

[gradio-pr-bot]

All the demos for this PR have been deployed at https://huggingface.co/spaces/gradio-pr-deploys/pr-3345-all-demos

[pngwn]

Looks good! Thanks @freddyaboulton!

[freddyaboulton]

Thank you for the review @pngwn ! 🐧