ENH Adds FeatureUnion.__getitem__ to access transformers

[thomasjpfan]

Reference Issues/PRs

Closes #24906

What does this implement/fix? Explain your changes.

This PR adds __getitem__ to FeatureUnion to access transformers.

[Micky774]

PR looks good modulo the test modification.

[betatim]

The __getitem__ of Pipeline supports slicing, which then returns a Pipeline containing only a subset of the steps. Worth implementing this here as well? If not, should we check the type of the argument to give an useful error message??

[thomasjpfan]

I do not think FeatureUnion.__getitem__ should support slicing or integers. I updated this PR to restrict the key to strings.

[betatim]

LGTM.

Fine with me not to support slicing. Can you write down a short summary of the thought process so people from the future (and for my education) can find out?

[thomasjpfan]

Fine with me not to support slicing. Can you write down a short summary of the thought process so people from the future (and for my education) can find out?

For me, I do not see a good user story for adding slices to FeatureUnion. For Pipeline, it is relatively common to do pipeline[:-1] to return a pipeline with only the transformers.

Logistically, I prefer to have a more restrictive feature and only consider expanding it when there is a need.

[jeremiedbb]

LGTM. Thanks