cudf groupby q6 median

[jangorecki]

median was added in cudf (rapidsai/cudf#2905), when it will get published (0.11) and we will move to this release, we should also add q6.

[jangorecki]

also we should use this rapidsai/cudf#2468 after upgrade to 0.11

[jangorecki]

upgrade to 0.11.0 did not solve this issue because median and std are not yet supported in groupby:
rapidsai/cudf#3429 rapidsai/cudf#3736
also required rapidsai/cudf#3737

[jangorecki]

@rnyak I am not developing cudf, just using it, please see linked issues for status and if needed ask question in cudf repo.
For now cudf is not even run on benchmark because it can very easily corrupt memory driver context. Once that happens machine needs a full reboot. For details see #129.
To address that I am waiting for cudf documentation of dask-cudf.

[jangorecki]

It was already implemented, should be released as 0.13.