Add post-processing steps to dask_cudf.groupby.CudfSeriesGroupby.aggregate

[charlesbluca]

Closes #8655

Adds some post-processing steps to Dask-cuDF's series groupby when using the optimized codepaths for aggregations, to match those done by Dask. These ensure that a dask_cudf.Series is always returned for the groupby operation, which was a problem observed in #8655.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (branch-21.08@a5427d2). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head 14bbd44 differs from pull request most recent head 185f96f. Consider uploading reports for the commit 185f96f to get more accurate results

@@               Coverage Diff               @@
##             branch-21.08    #8694   +/-   ##
===============================================
  Coverage                ?   10.67%           
===============================================
  Files                   ?      109           
  Lines                   ?    18667           
  Branches                ?        0           
===============================================
  Hits                    ?     1993           
  Misses                  ?    16674           
  Partials                ?        0           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a5427d2...185f96f. Read the comment docs.

[charlesbluca]

@gpucibot merge