Remove TargetTransformer class

[bchen1116]

fix #2687

[codecov]

Codecov Report

Merging #2833 (ec40190) into main (20a00c8) will not change coverage.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main   #2833   +/-   ##
=====================================
  Coverage   99.8%   99.8%           
=====================================
  Files        297     297           
  Lines      27741   27741           
=====================================
  Hits       27664   27664           
  Misses        77      77           

Impacted Files	Coverage Δ
...l/pipelines/components/transformers/transformer.py	100.0% <ø> (ø)
evalml/pipelines/component_graph.py	99.8% <100.0%> (-<0.1%)	⬇️
...ents/transformers/preprocessing/log_transformer.py	100.0% <100.0%> (ø)
...transformers/preprocessing/polynomial_detrender.py	97.7% <100.0%> (+0.2%)	⬆️
evalml/tests/component_tests/test_components.py	99.3% <100.0%> (-<0.1%)	⬇️
...valml/tests/pipeline_tests/test_component_graph.py	99.9% <100.0%> (+0.1%)	⬆️

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[angela97lin]

Love these cleanup PRs, keep them coming 😂 ! LGTM, thank you! 👍

[angela97lin]

Open question: Do you think this will hold true long term? It's true for our components now but... 👀

[bchen1116]

Hmm, I do think that if we have an inverse_transform method, then it'll modify the target rather than the features. It doesn't seem to make sense to have a transformer have an inverse_tranform method for the features. I also don't think it'd make sense to introduce an inverse_transform method if we don't alter the target at all. Logically, it seems like this should still hold long term.

[freddyaboulton]

Should we add inverse_transform as a method to Transformer? The "default" behavior is a no-op and if we want to enforce that transformers that modify the target override that behavior we can do something like

def inverse_transform(self, y):
    if self.modifies_target:
        raise NotImplementedError("Transformers that modify the target must override inverse_transform")
    return y

That way this behavior always holds. Or we don't make this change? Maybe I was being crazy when I filed the issue lol

[chukarsten]

I think Freddy's suggestion is a good one!

[chukarsten]

LGTM!