Exposing transformations applied by Cleaner and TableVectorizer

[emassoulie]

Closes #1265
Adds a method to the Cleaner and TableVectorizer transformers enabling them to give a human-readable list of the transformations (DropUninformative, ToFloat, ToDatetime) they are set to apply once fitted.

[emassoulie]

I've got something going with the transformation lists for both transformers! I feel the TableVectorizer transformations could be equally relevant to expose by raw transformer type (ToFloat, ToDatetime etc, like the Cleaner), or by transformer category ("numeric", "low cardinality" etc, since they are customisable). I'm thinking of going towards the latter, but I wanted to ask for a second opinion before I go ahead.

[emassoulie]

It's starting to come together! The direction chosen is to output a string viewable in any terminal, and to list columns grouped by transformation. On this second point, I've been having thoughts on doing it the other way, adding a way of describing the pipeline each individual column goes through: does that seem relevant or does the current way work fine?

I've attached the output to TableVectorizer.list_transformations() after running fit on two different examples, one on the employee dataset and one on the custom-made dataset created for the test. Does it look like the kind of output we want?

[rcap107]

Thank you very much for the PR @emassoulie ! I did a review of the current material.

In general, I think we're on the right path. I think the main thing to decide is the actual format of the returned string. I think there is too much whitespace, so we may want to investigate how other symbols may make it easier to parse the output.

Other than that, good job!

[rcap107]

Thanks a lot for improving the PR!

I think we're getting close to the end, though I think the code can still be improved and simplified in places.

I opened a PR targeting this branch in emassoulie#2 to propose some changes in that direction

Could you also add a mention of list_transformations in the docstring of TableVectorizer and Cleaner? Without printing the entire thing, only the first few lines followed by ... to elide the rest