Add list output option to character_ngrams() function

[davidwendt]

Closes #8190

Depends on #9498

Adds an as_list output option to the cudf character_ngrams() API. The generated ngrams can be grouped within list offsets.

Example 1

import cudf
s = cudf.Series(["bumblebee", "transformers"])
print(s.str.character_ngrams(as_list=True))

0                [bu, um, mb, bl, le, eb, be, ee]
1    [tr, ra, an, ns, sf, fo, or, rm, me, er, rs]
dtype: list

Strings too small for the ngram size result in empty list rows.
And as always, null rows result in null rows.

Example 2

import cudf
s = cudf.Series(["bumblebee", "transformers", None, "", "a"])
print(s.str.character_ngrams(as_list=True))

0                [bu, um, mb, bl, le, eb, be, ee]
1    [tr, ra, an, ns, sf, fo, or, rm, me, er, rs]
2                                            None
3                                              []
4                                              []
dtype: list

[shwina]

I believe mypy complains here because the return type of self.len() is a Union[Series, BaseIndex], and BaseIndex does not support a __sub__() method.

@vyasr do you have any thoughts on how to resolve? (other than a type: ignore of course :))

[vyasr]

Is the typing really accurate? Shouldn't it return Series for Series of strings or Int64Index for any index type? This seems like an opportunity to tighten up our annotations.

[codecov]

Codecov Report

Merging #9499 (3e46136) into branch-21.12 (ab4bfaa) will decrease coverage by 0.12%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##           branch-21.12    #9499      +/-   ##
================================================
- Coverage         10.79%   10.66%   -0.13%     
================================================
  Files               116      117       +1     
  Lines             18869    19723     +854     
================================================
+ Hits               2036     2104      +68     
- Misses            16833    17619     +786     

Impacted Files	Coverage Δ
python/dask_cudf/dask_cudf/sorting.py	92.90% <0.00%> (-1.21%)	⬇️
python/cudf/cudf/io/csv.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/hdf.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/orc.py	0.00% <0.00%> (ø)
python/cudf/cudf/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/_version.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/abc.py	0.00% <0.00%> (ø)
python/cudf/cudf/api/types.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/dlpack.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
... and 66 more

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[galipremsagar]

@davidwendt Your examples are always amazing

[galipremsagar]

@gpucibot merge