Remove 10.2 workarounds in groupby functions for dictionary column types

[davidwendt]

Reference #7913
This removes the compiler restrictions in the libcudf groupby code that was disabled because of compiler issues on nvcc 10.2 when adding dictionary columns support. The corresponding disabled gtests have been enabled as well.

Details of the compiler bug are documented here: https://nvbugswb.nvidia.com/NvBugs5/SWBug.aspx?bugid=3186317

This also fixes an issue where the sum-of-squares aggregation was not working for values specified as a dictionary column.

[codecov]

Codecov Report

Merging #7949 (394e424) into branch-0.20 (51336df) will decrease coverage by 0.04%.
The diff coverage is 85.37%.

@@               Coverage Diff               @@
##           branch-0.20    #7949      +/-   ##
===============================================
- Coverage        82.88%   82.84%   -0.05%     
===============================================
  Files              103      103              
  Lines            17668    17804     +136     
===============================================
+ Hits             14645    14750     +105     
- Misses            3023     3054      +31     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/__init__.py	100.00% <ø> (ø)
python/cudf/cudf/io/orc.py	86.89% <ø> (ø)
python/cudf/cudf/utils/cudautils.py	57.75% <25.00%> (ø)
python/cudf/cudf/utils/dtypes.py	81.87% <41.66%> (-1.57%)	⬇️
python/cudf/cudf/core/column/numerical.py	94.43% <72.72%> (ø)
python/cudf/cudf/core/tools/datetimes.py	80.25% <74.69%> (-4.28%)	⬇️
python/cudf/cudf/core/groupby/groupby.py	91.55% <76.92%> (+0.11%)	⬆️
python/cudf/cudf/core/column/column.py	88.64% <77.77%> (ø)
python/dask_cudf/dask_cudf/backends.py	89.51% <80.00%> (-0.08%)	⬇️
python/cudf/cudf/core/column/decimal.py	90.83% <84.84%> (-2.09%)	⬇️
... and 25 more

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[davidwendt]

rerun tests

[karthikeyann]

Creating separate functor for each operation for dicitonary keytype is repeating code and not scalable. It needs another refactor, may be in another PR. For removing 10.2 workarounds, this PR looks good.

[ttnghia]

Rerun tests.

[harrism]

@gpucibot merge