Return X and y from every transformer

In order to build transformers that transform the target variable, i.e. #1944 and #914, our all of our transformers must return both `X` and `y` for the target transformations to make it all the way down to the target.

Note, I think we've already made the design choice that transformers [must take in both `X` and `y`](https://github.com/alteryx/evalml/blob/main/evalml/pipelines/component_graph.py#L208) and [return `X` and `y`](https://github.com/alteryx/evalml/blob/main/evalml/pipelines/component_graph.py#L211) in our DAG `ComponentGraph` but we haven't made that clear in our transformer api.

This issue tracks implementing this design choice in our transformer api.