MAINT Fix builder partial fit

[adam2392]

Reference Issues/PRs

Fixes state of builder_ to not need to be maintained.

Prolly needs unit-tests to determine if this "functions as desired". I.e.

• changing datatype of X over multiple partial fits should fail nicely,
• changing datatype of y
• classification and regression

What does this implement/fix? Explain your changes.

Any other comments?

[github-actions]

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