Refactor imports from preprocessing to steps

[noahho]

Issue

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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 changelog entry has been added (see changelog/README.md), or "no changelog needed" label requested.
• The code follows the project's style guidelines.
• I have considered the impact of these changes on the public API.

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[gemini-code-assist]

Code Review

This pull request refactors imports to be more specific, changing from tabpfn.preprocessing to tabpfn.preprocessing.steps. The change is correct and improves code organization. I've added one suggestion to use the inspect module for class discovery, which is a more standard and readable approach for introspection.

[Copilot]

Pull request overview

This PR refactors the import statement in the test file to correctly reference the steps module within tabpfn.preprocessing, ensuring that the _get_preprocessing_steps() helper function can properly iterate over the preprocessing step classes.

Key Changes:

• Updated import from from tabpfn import preprocessing to from tabpfn.preprocessing import steps
• Changed iteration target from preprocessing.__dict__.values() to steps.__dict__.values() to correctly access step classes

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