Fix base_values in TreeEnsembleRegressor

[corwinjoy]

Description

In the reference implementation of TreeEnsembleRegressor when a value is provided for the base_values argument, this value replaces any prediction. This can't be what's intended and does not match onnxruntime. Instead, the base_value should be added to the prediction after applying any aggregation.

Motivation and Context

We are exporting regression trees into ONNX that have a non-zero base_value as the baseline prediction for the tree. Prediction works as expected in onnxruntime but not in the reference implementation. I believe this is an oversight and propose the fix (plus tests) below. I also think the documentation should be more explicit about what the base_values argument does.

[justinchuby]

Please fix the lint errors. Thanks!

[gramalingam]

Thanks for the fix!

[corwinjoy]

Lint errors should be fixed now according to local lintrunner.

[justinchuby]

I would use itertools.product for this. A double for loop in list comprehension can be confusing: https://google.github.io/styleguide/pyguide.html#274-decision

[corwinjoy]

@justinchuby I don't know how to do this in a clear way with itertools.product because of the name variable at the beginning. I do want to keep the name variable, since I think that is the whole point of the parameterization - that is to more clearly see any failures directly from the test name.

[justinchuby]

Could you rebase on main to remove the relevant commits? Thanks!

[corwinjoy]

I tried a rebase on main but it still shows all the other merges / updates as commits. Not sure how to fix. It may require a new branch / PR? The signed commits + rebasing the code seems to be creating problems.

[justinchuby]

I tried a rebase on main but it still shows all the other merges / updates as commits. Not sure how to fix. It may require a new branch / PR? The signed commits + rebasing the code seems to be creating problems.

I usually do this: branch this out to a new branch, reset this current branch to upstream/main, then squash merge the new branch to this branch; force push.