add TF-IDF Transformer

[pr38]

Hi,

I am trying to add TfidfTransformer to dask. I am going to assume that adding TfidfVectorizer can be easily done later down the line, by stacking CountVectorizer and TfidfTransformer .

This is not the first attempt to add tfidf to dask-ml(https://github.com/dask/dask-ml/pull/869/files#diff-421c76129d7900a3ae83237eab5785858c0652d9f550d92e0202e45ebdcb2977). I can't comment on the differences between my attempt and previous attempts; but I can guaranty that my implementation only use's the dask array api, no other api.

[stsievert]

Thanks for the contribution! At first glance, this looks pretty good. I'd appreciate some more review.

All these comments and linting errors are minor. If you want to run the linting yourself, look at https://ml.dask.org/contributing.html#style

[pr38]

ok, I am ready for more feedback.

I should mention that I am running compute_chunk_sizes when the number of rows are not know. I have seen maintainers complain about other's doing that here. If you guys don't want me to run compute_chunk_sizes , maybe we can make sure that CountVectorizer keeps chunk_sizes, to avoid foot guns.

[pr38]

There are a couple of extra things I want to point out. While scikit-learn returns a scipy.sparse.coo_array,I return a sparse.COO. I don't know how well the rest of dask-ml plays with sparse.COO, and I am not sure how committed to 1 to 1 matching scikit-learn dask-ml is.

I have also seen two other implementations of '_handle_zeros_in_scale' in the dask-ml code base. I would have used the existing implementations but none used epsilon to catch near zero values.

[stsievert]

You've also got some style errors. Could you look at run the linting commands at https://ml.dask.org/contributing.html#style ?