Implement a mixin for reductions

[vyasr]

This PR implements a factory for mixins classes based on the common pattern in cuDF of categories of similar functions all calling a common method implementing some standard pre/post-processing before calling a lower-level API of either one of its members (e.g. Frames calling Column methods) or the C++ libcudf library. When added to another class, these mixins support customization of which methods are exposed via a class member set of method names. Documentation for these methods is generated by formatting the docstring for the common internal method, e.g. _reduce for reductions. As a first pass, this PR generates a single mixin for reductions and applies it to all the relevant classes. Future PRs will use this to generate classes for scans, binary operations, and unary operations, and perhaps other similar categories as they are uncovered.

This approach assumes a great deal of API homogeneity between the different methods in a category. Frame violates this assumption because similar operations often support slightly different parameters (for instance, some reductions support a min_count parameter), so for now Frame was not made Reducible. That decision could be revisited if 1) the degree of homogeneity of these function signatures increases over time, or 2) we can introduce greater customization into these mixins without adding too much complexity. A first attempt of (2) can be seen in this branch, but the degree of additional complexity just to support Frame isn't really justifiable at this stage, so unless we can come up with a simpler solution I recommend leaving Frame as is for now.

[shwina]

Marking this DO NOT MERGE, as @vyasr and I agreed we would like to first see how well the approach taken here generalizes beyond reductions, by trying it with something like binary ops or scans. That will be in a separate branch.

[isVoid]

Aside from small docstring improvement, I have one thought in comment, but probably doesn't require any actions.

Other thoughts: It certainly require some training to be able to use the factory. There are subtle things like op parameter requires a annotation, and the docstring of the _base_operation is required to exist. I wish we can make both optional in compliance with python standards.

Superseded by the latest review.

[shwina]

There are subtle things like op parameter requires a annotation, and the docstring of the _base_operation is required to exist. I wish we can make both optional in compliance with python standards.

Definitely something we need to address. python -OO for example fails for me with this branch because it explicitly discards docstrings:

$ python -OO
Python 3.8.12 | packaged by conda-forge | (default, Jan 30 2022, 23:42:07)
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import cudf
>>> df = cudf.DataFrame()
>>> df.groupby([]).max()  # AttributeError

[vyasr]

There are subtle things like op parameter requires a annotation, and the docstring of the _base_operation is required to exist. I wish we can make both optional in compliance with python standards.

Definitely something we need to address. python -OO for example fails for me with this branch because it explicitly discards docstrings:

$ python -OO
Python 3.8.12 | packaged by conda-forge | (default, Jan 30 2022, 23:42:07)
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import cudf
>>> df = cudf.DataFrame()
>>> df.groupby([]).max()  # AttributeError

Yeah this is just an oversight on my part. Optional attributes should only be patched if they are defined.

[vyasr]

Marking this DO NOT MERGE, as @vyasr and I agreed we would like to first see how well the approach taken here generalizes beyond reductions, by trying it with something like binary ops or scans. That will be in a separate branch.

@isVoid @shwina An example of this is in vyasr#3

[isVoid]

Aside from docstring example: #9925 (comment), just a few parting thoughts here.

[isVoid]

Actually I meant to click approve. I think the above question/comments does not count as blockers.

[vyasr]

@gpucibot merge