[ENH] Add CovariateRegressor class

[chiuhoward]

With some assistance from @richford

• Implements CovariateRegressor for regressing covariates from features
• Includes find_subset_indices utility for hash and precise matching
• Adds IdentityTransformer helper class
• Tests cover edge cases and error handling
• Supports cross-validation, missing value imputation, and flexible pipelines
• All tests pass with full coverage of core functionality

[arokem]

I believe the test failure is unrelated. It might be an update to pytorch API that I need to investigate, before we can have your code run cleanly.

[chiuhoward]

@arokem I believe that it’s because verbose was deprecated from ReduceLROnPlateau at some point? I encountered the same failing tests earlier in the year when I tried to just fix a typo in the docs.

pytorch/pytorch#111302

[arokem]

@chiuhoward : do you want to make a separate PR just to fix that failure first, and then we can move on with this one?

BTW: I gave it a first read through and the code in this PR looks great. I will give it a thorough review once we resolve the test failures, if that's OK with you.

[arokem]

OK - now that #29 is merged, you can either rebase this one on top of main, or merge main into this branch and then push again, so we can see that all tests are passing with that fix.