Enforce boolean ascending for dask-cudf sort_values

[charlesbluca]

It is possible to pass a list of ascending booleans to dask-cudf's sort_values, which is not yet supported by quantile_divisions (which computes divisions for all sort-by columns sorted in ascending order), and can cause undefined behavior.

This small PR adds a check that ascending is a boolean before computing quantile divisions; note that this check happens after the single-partition case is handled, as cuDF can handle lists of ascending booleans.

[codecov]

Codecov Report

Merging #9814 (951c3dc) into branch-22.02 (967a333) will decrease coverage by 0.35%.
The diff coverage is 9.91%.

@@               Coverage Diff                @@
##           branch-22.02    #9814      +/-   ##
================================================
- Coverage         10.49%   10.13%   -0.36%     
================================================
  Files               119      119              
  Lines             20305    21557    +1252     
================================================
+ Hits               2130     2185      +55     
- Misses            18175    19372    +1197     

Impacted Files	Coverage Δ
python/cudf/cudf/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/column.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/string.py	0.00% <ø> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/groupby/groupby.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/index.py	0.00% <ø> (ø)
python/cudf/cudf/core/indexed_frame.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/multiindex.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/utils.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/sorting.py	92.30% <50.00%> (-0.61%)	⬇️
... and 8 more

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[galipremsagar]

rerun tests

[galipremsagar]

@gpucibot merge