Suggest var_names when using deprecated API for partial traces

[lancechua]

Description

Sampling a partial trace was first done in #271, with the trace kwarg.
Then, it subsequently removed in #6269 raising DeprecationWarning error.
Partial trace was added back with the var_names kwarg in #7206.

This PR updates the error message to reference the new var_names kwarg instead, as trace may still be suggested by coding LLMs.

Related Issue

• Closes #
• Related to sample() lacks track argument #271 Simplify mcmc module #6269 Add var_names argument to sample #7206

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

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📚 Documentation preview 📚: https://pymc--7289.org.readthedocs.build/en/7289/

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[lancechua]

An alternative is var_names = trace if isinstance(trace, list) else var_names, but that would violate the one-way-to-do-it "rule"...

[ricardoV94]

Thanks @lancechua, this seems fine

[michaelosthege]

Thanks @lancechua !

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