[BUG] Add BaseEstimator to TargetConfoundRemover

[LeSasse]

Describe the bug
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Currently, the TargetConfoundRemover is not a BaseEstimator, hence i get a fit_failed, because sklearn does not like it when you can't get_params().

To Reproduce
Steps to reproduce the behavior:
run_cross_validation(
...
preprocess_y="remove_confound"
...
)

Expected behavior
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Remove confounds from the target.

Screenshots
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System (please complete the following information):

• Linux

Additional context
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[LeSasse]

Forgot to tag @samihamdan