Adds a placeholder argument to gr.Chatbot

[abidlabs]

I had previously created a custom component with this functionality but after discussion with @dawoodkhan82 figured it'd be useful to have in core.

Here's what it looks like:

import gradio as gr

placeholder = """
<img src="https://avatars.githubusercontent.com/u/51063788?s=400&u=479ecc9d93d8a373b5c2e69ebe846f394811e94a&v=4)" style="width:15%">

**Gradio Helper**

This Chatbot can help you on any topic related to Gradio.
"""

gr.ChatInterface(lambda x,y:x, chatbot=gr.Chatbot(placeholder=placeholder)).launch()

The placeholder only appears when there are no messages to display.

[gradio-pr-bot]

🪼 branch checks and previews

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---

Install Gradio from this PR

pip install https://gradio-builds.s3.amazonaws.com/a2730b77daa58af7f65bad580fe8241ed90e8659/gradio-4.23.0-py3-none-any.whl

Install Gradio Python Client from this PR

pip install "gradio-client @ git+https://github.com/gradio-app/gradio@a2730b77daa58af7f65bad580fe8241ed90e8659#subdirectory=client/python"

[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
@gradio/chatbot	minor
gradio	minor

• Maintainers can select this checkbox to manually select packages to update.

With the following changelog entry.

Adds a placeholder argument to gr.Chatbot

Maintainers or the PR author can modify the PR title to modify this entry.

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• Maintainers can change the version label to modify the version bump.
• If the bot has failed to detect any changes, or if this pull request needs to update multiple packages to different versions or requires a more comprehensive changelog entry, maintainers can update the changelog file directly.

[dawoodkhan82]

Nice, LGTM!

[pngwn]

Looks good! Thanks @abidlabs!