Allow string aggs for dask_cudf.CudfDataFrameGroupBy.aggregate

[charlesbluca]

I noticed that CudfDataFrameGroupBy.aggregate doesn't actually support passing aggregations as strings, for example something like

import cudf
import dask_cudf

gdf = cudf.DataFrame({'id4': 4*list(range(6)), 'id5': 4*list(reversed(range(6))), 'v3': 6*list(range(4))})
gddf = dask_cudf.from_cudf(gdf, npartitions=5)

gddf.groupby("id4").agg("mean")

Would actually end up using the upstream aggregate implementation. This is because:

• CudfDataFrameGroupBy.aggregate does not convert string aggs to a dict before calling _is_supported on them
• _is_supported only handles list / dict aggs, returning false otherwise

I've resolved this by adding string support to _is_supported, and moving the conversion of aggs to the internal groupby_agg.

It looks like this is exposing some failures for first and last groupby aggs, as tests that were originally using upstream Dask to compute these aggregations (I assume accidentally since these aggregations are listed as supported) are now using dask-cuDF and getting the wrong result.

[github-actions]

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[codecov]

Codecov Report

Merging #10222 (a54ee42) into branch-22.06 (e0d94f3) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@               Coverage Diff                @@
##           branch-22.06   #10222      +/-   ##
================================================
+ Coverage         86.28%   86.32%   +0.03%     
================================================
  Files               144      144              
  Lines             22654    22656       +2     
================================================
+ Hits              19548    19558      +10     
+ Misses             3106     3098       -8     

Impacted Files	Coverage Δ
python/cudf/cudf/core/resample.py	88.97% <ø> (ø)
python/dask_cudf/dask_cudf/groupby.py	97.42% <100.00%> (+0.01%)	⬆️
python/dask_cudf/dask_cudf/tests/test_groupby.py	100.00% <100.00%> (ø)
python/cudf/cudf/core/dataframe.py	93.78% <0.00%> (+0.04%)	⬆️
python/cudf/cudf/core/column/string.py	88.78% <0.00%> (+0.12%)	⬆️
python/cudf/cudf/core/groupby/groupby.py	91.79% <0.00%> (+0.22%)	⬆️
python/dask_cudf/dask_cudf/core.py	73.62% <0.00%> (+0.26%)	⬆️
python/cudf/cudf/core/column/numerical.py	96.17% <0.00%> (+0.29%)	⬆️
python/cudf/cudf/core/tools/datetimes.py	84.49% <0.00%> (+0.30%)	⬆️
... and 1 more

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[bdice]

No comments -- looks good to me!

[galipremsagar]

@gpucibot merge