Use data models in preprocess()

[abidlabs]

Picking up #6127

As discussed internally with @freddyaboulton, this uses data models in the .preprocess(), not just the .postprocess()

This PR makes the following changes:

• Removes the serializes function: https://github.com/gradio-app/gradio/blob/v4/gradio/component_meta.py#L59 as FastAPI automatically converts Pydantic models into JSON
• Pre-Preprocesses incoming payload from the frontend to convert to Pydantic data models before sending to .preprocess()
• Cleans up the code as a result of the above changes, adds typing to missing places, etc.
• Rename x to payload in preprocess() to signify that its the payload from the frontend
• Rename y to value in postprocess() to signify its the same type as the value of the component
• Removes docstrings from preprocess() and postprocess() because many were incorrect / outdated / rudimentary. The typing is better than the docstrings.

I think it would be better to merge this after @pngwn's #6094 PR to avoid introducing many backend conflicts to that PR.

Closes: #1798

[gradio-pr-bot]

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Install Gradio from this PR

pip install https://gradio-builds.s3.amazonaws.com/69680e2bf72e0b01e17a867ed58d98a602e153a0/gradio-3.50.2-py3-none-any.whl

Install Gradio Python Client from this PR

pip install "gradio-client @ git+https://github.com/gradio-app/gradio@69680e2bf72e0b01e17a867ed58d98a602e153a0#subdirectory=client/python"

[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
gradio	minor
gradio_client	minor

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Use data models in preprocess()

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[abidlabs]

This PR is becoming too big -- I'm going to break it up into smaller pieces and try again