fix: return the sum of squared errors

[LouisDeconinck]

Describe your change

sum_of_square_error was documented as returning the sum of squared errors, but it divided that sum by 2 * len_data, returning half the mean squared error instead. This made the regression error report mathematically incorrect. The fix returns the actual sum of squared errors and removes the now-unused length argument.

Fixes #8847

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests?
• Documentation change?

Checklist

• I have read CONTRIBUTING.md.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All functions in the changed file have passing doctests.
• The change is limited to the existing algorithm and its doctest.