Add usage tips for TabPFN in README

[noahho]

Added important tips for using TabPFN effectively, including batch prediction mode, data preprocessing advice, GPU usage, and dataset size limitations.

Issue

Please link the corresponding GitHub issue. If an issue does not already exist, please open one to describe the bug or feature request before creating a pull request.

This allows us to discuss the proposal and helps avoid unnecessary work.

Motivation and Context

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Public API Changes

• [ ] No Public API changes
• [ ] Yes, Public API changes (Details below)

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How Has This Been Tested?

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Checklist

• [ ] The changes have been tested locally.
• [ ] Documentation has been updated (if the public API or usage changes).
• [ ] A entry has been added to CHANGELOG.md (if relevant for users).
• [ ] The code follows the project's style guidelines.
• [ ] I have considered the impact of these changes on the public API.

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[noahho]

/gemini review